If you're breastfeeding a baby with food allergies, you may have been told that it takes 2 weeks for food proteins to get out of your breast milk. But is this really true? In today's guest post, Dr Trill, a Molecular Biologist who has breastfed two babies with severe food allergies, tell us what the science really says about how long allergens stay in your breast milk:
Breastfeeding with Food Allergies
Few things are as terrifying as an infant allergic reaction. It can come in the form of a rash, chronic diarrhea (possibly with blood), anaphylaxis, and more. Food sensitivities can appear as early as the first few weeks of life. Such a young age, combined with these severe symptoms, can leave parents desperate for answers. I was in this situation three years ago, bawling in the hospital stairwell after doctors told me that my daughter was in so much pain because of my breast milk.
We discovered that my 3-week-old had severe food sensitivities. Since traditional allergy testing is ineffective for children that young, we were informed that it was best to eliminate all allergens from her diet. We were strongly encouraged to end our breastfeeding journey and replace it with an amino acid formula that smelled like sweaty gym socks and cost $50 a CAN.
I wanted so badly to continue breastfeeding my daughter. The doctors stated that I could do so but would have to remove all allergens from my diet for at least two weeks before I could feed her. According to them, that was the amount of time it would take for the food proteins to leave my breast milk. During those two weeks, we fed her the formula while I pumped around the clock. Finally, two weeks passed, and we were incredibly lucky that the transition back to breastfeeding was smooth (probably because it was an improvement from sweaty gym socks).
While I was grateful to be breastfeeding again, I lived every day in fear. Fear that I would read a label wrong. Fear that I would run out of my packed meals and need to grab something on the go. Fear that my husband would forget and accidentally add the wrong ingredient to my food. Debilitating fear. Every time there was a fussy night or a tiny bump on her body I would die on the inside, thinking: I did that. She’s having a reaction and it is all my fault.
How Long do Allergens Stay in Breast Milk?
Since then, I graduated with my Ph.D. in Cellular and Molecular Biology. So, when my second daughter was born with the same issues, I decided to dig into the research. What I found was SHOCKING.
- It typically takes 2-8 hours for breast milk to be clear of food allergens after ingestion.
- Ingested food allergens do not always make their way to breast milk.
Mind. Blown. Scientists have conducted several studies of breast milk at different time intervals after ingestion by the mother. For instance, one lab gave 23 women a serving of peanuts. Only half of them ended up with detectable levels of peanut protein in their milk. The milk that did contain the allergen peaked in concentration between 1-2 hours and steadily decreased from there [1].
So why didn't my doctor tell me this?
The question emerges: WHY would doctors say it takes 2 weeks? WHY would online articles spread this lie?
I believe the answer is two-fold. First, scientific research explains that it may take up to 2 weeks for the infant to be clear of symptoms. This is the case whether the child is placed on hypoallergenic formula OR breastmilk in conjunction with an elimination diet (let’s call this hypoallergenic breast milk). So, while mothers may be clear of allergens, their infant will likely continue to show symptoms for a few more weeks.
After speaking to a few MDs about this, they all made the same basic statement: they are taught to say that allergens exist in milk for up to two weeks as a precaution and to explain why the child is still suffering from symptoms. The extra precaution is the second element. Every person’s metabolism is different so the exact amount of time that it will take any given allergen to clear from milk will vary. However, the research undoubtedly shows it is nowhere near two weeks.
How about online articles?
Even Kellymom.com makes this statement:
If you think your baby is reacting to a particular food, then eliminate that food from your diet for 2-3 weeks to see if baby’s symptoms improve. If baby’s symptoms do improve, then this food may be a problem for your baby. Eliminating a food for less than 2-3 weeks may not be effective—cow’s milk protein, for example, can persist in mom’s body for 1½ – 2 weeks, and it may be another 1½ – 2 weeks before the protein is out of baby’s system.
However, if you dig into their cited research you will find a review which references a study conducted by Dr. Nowak [2] which makes the following statements:
“In the majority of infants with [cow's milk] and/or soy FPIES, symptoms resolve within 3–10 days of starting extensively hydrolyzed casein formula.”
“Symptomatic infants with chronic FPIES improve within 3–10 days with i.v. fluids or with hypoallergenic formula.”
Nowhere in this cited journal does it say anything about causing a continuation of symptoms through contaminated breast milk that lasts for 2 weeks.
It’s important to note that I’m not talking about “pumping and dumping”. You will naturally send the protein through your body, to potentially include your milk, and then your cellular metabolism will use it up [3]. Your milk is not a holding place where things can enter from the body and only exit through your nipples. Breast milk is constantly changing, no dumping required.
What does this mean?
It means you can finally breathe. If you are just starting your elimination diet for your food sensitive infant, it will take less than a day to produce hypoallergenic breastmilk. If you accidentally eat an allergen, you could wait to continue breastfeeding for up to 8 hours and then confidently power on. It would be wise to have at least 24 hours’ worth of saved breastmilk or formula available in case this occurs. Depending on how much of the allergen you ate and how severe the symptoms, you can decide when you feel comfortable nursing again. During that time, you can pump and donate the “allergen-contaminated” milk to a baby without allergies. No one is perfect, and the fact that you do not have to be, should be more widespread in the breastfeeding, allergen, and medical worlds.
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Update 11th May 2020: There is a further study, which shows that cow's milk protein can be detected in breast milk 7 days after eating a single serving of cow's milk. However, the *highest* concentration of cow's milk protein detected in this study was 200 times lower than the amount needed to cause a reaction. So there is no need for breastfeeding mums to worry. Read Dr Trill's comments on this in her blog post, and read the original study here: https://www.ncbi.nlm.nih.gov/pubmed/28222656
References:
- Vadas, Peter, et al. "Detection of peanut allergens in breast milk of lactating women." Jama 285.13 (2001): 1746-1748.
- Nowak-Węgrzyn A. Food protein-induced enterocolitis syndrome and allergic proctocolitis. InAllergy and asthma proceedings 2015 May (Vol. 36, No. 3, p. 172). OceanSide Publications.
- Kilshaw PJ, Cant AJ. The passage of maternal dietary proteins into human breast milk. International Archives of Allergy and Immunology. 1984;75(1):8-15.
About Dr. Trill
Trillitye Paullin, Ph.D.
Trill’s passion as a Molecular Biologist and mother of two led to her founding Free to Feed, LLC. F2F creates science-based content for allergen-free breastfeeding and is developing products to empower breastfeeding parents. Her current position, overseeing a dairy processing laboratory, has allowed her to translate knowledge in the dairy industry to the benefit of parents with food-sensitive infants. In addition to her professional civilian career, Trill is an OIF Veteran and is presently serving her 13th year in the Army National Guard.
Eva
Hello Doctor!
You said, "it typically takes 2-8 hours for breast milk to be clear of food allergens after ingestion." May I ask if this also applies to medication and herbal supplements?
Thanks!
Gloria Thai
The time it takes for medications and herbal supplements to clear varies. Contact Infant Risk. Dr. Thomas Hale is the expert on medications and mother's milk.
Zoe
Hi Eva! Thank you for your comment. It's a great question! I've asked Dr Trill to pop over and answer this question (as I'm not a scientist myself!). In the meantime have a look at this article from Kellymom which should give you some more information https://kellymom.com/hot-topics/med-risks/
Kayla Holland
Thank you so much for sharing this information. I have been beating myself up, trying to figure out what on earth is contributing to/causing my son's never ending diaper rash (which his doctor said was probably eczema) and decided to start eliminating allergens from my diet. Every time it flares back up, I've been assuming I somehow missed an allergen ingredient or did something wrong, but looking over food logs never shows anything, I've been operating under the assumption that the "it can take up to 4 weeks for cow's milk to stop affecting baby," because that's what I was told, and feeling guilty for things I was still eating weeks ago. This is amazing info to have. Thank you!
Victoria
So what was causing the diaper rash? I’m dealing with a diaper rash that keeps coming back despite very frequent changes, washing with soap and water every changing. Blow drying, and using diaper cream every time. It doesn’t make sense and in desperate.
Lauren
I know this is a late response, but have you tried changing diaper and/or wipes brands? My little one kept getting rashes despite frequent changes/cleaning/etc, all the right things - we eventually discovered it was a reaction to his diapers. Even some “sensitive” diapers can sadly irritate his skin. We found Seventh Generation, Honest, Millie Moon, and Costco’s Sensitive diapers work for him, but I’m sure each kid is different! Best of luck!
Lindsey
I just recently switched from cows milk to almond milk and almond flour instead of regular as part of my attempt to lose weight. My daughter has had a severe hive reaction so i didn’t breastfeed for about eight hours and i was doing research and i found this article so after about nine hours i decided to go ahead and nurse with her. She immediately started getting the hives again and this time they were worse than the time right after i ate the almond product! I’m more lost than i was before..
Zoe
Please discuss this with your child's doctor as soon as possible. It sounds like your child may have a nut allergy.
Renee
Thank you for thoroughly and accurately answering my question. This should have been the top result when I searched on google. I slipped up and ate a few bites of ice cream from my toddler’s unfinished cone (I know - the worst!) and my usually happy-go-lucky baby was screaming his head off at bedtime. I knew it was the dairy! I felt so bad seeing him in so much pain. I need to like tattoo this in my brain: ITS NEVER WORTH IT! I have been avoiding lactose since he was born. Goats milk and cheese is actually fine for him though.
Louise Carson
Hello, my son has been diagnosed with a milk protein allergy so I need to elimate soy and dairy from my diet. They told me to continue breastfeeding in the meantime and we will start to see some improvements within 2-3 weeks. For us it's so hard to see him in pain and are wondering if we should formula feed until my milk is clear of the allergens? They told us at the hospital to continue with breastfeeding but I hate seeing my baby in so much pain. Thanks in advance.
Zoe
Hi Louise, sorry to hear your little one is suffering with milk protein allergy. Your milk should be clear of milk protein quite quickly, but 2-3 weeks is how long it may take for his gut/skin to start healing. So you shouldn't need to formula feed during this time. Formula feeding can also have a negative effect on your milk supply and make it more difficult for you to resume breastfeeding later. Hope he feels better very soon.
Rachael Holmes
If goats or over animal milk doesn't affect baby it is not lactose allergy but probsbly cows milk protein allergy (CMPA) just wanted to give you this into to help your future journey.
Zoe
Hi Rachael, goat's milk and other animal milk isn't recommended for babies with CMPA because there is a high chance they will react to that as well: https://www.bjfm.co.uk/an-update-on-cows-milk-protein-allergy-and-best-practice-in-primary-care#:~:text=Goat's%20milk%20formula%20(e.g.%20Holle,risk%20of%20allergenic%20cross%2Dreactivity.&text=They%20are%20therefore%20not%20suitable%20for%20treating%20CMPA.
It's also not suitable for people with lactose intolerance because all animal milk contains lactose.
Kate
Hey!
Just wondering,
I’ve got a 13 month old who’s allergic to eggs, dairy and peanuts. We found this out over 8 months ago and have had more testing done which has revealed they aren’t getting better, they’re actually a bit more intense.
Anyways, I haven’t had even so much as a whiff of eggs, dairy or peanuts in over 8 months, and I really miss eggs. How long it would take the egg to leave my breastmilk? I normally feed him once in the morning, mainly for immunities as he’s just started daycare.
Thank you
Zoe
Hi Kate, I feel your pain, I breastfed my daughter with allergies to milk, soya, egg, wheat, oats and legumes until she was just under 2 years old and boy did I miss eating all those foods!
As Dr Trill states in the article "If you accidentally eat an allergen, you could wait to continue breastfeeding for up to 8 hours and then confidently power on. It would be wise to have at least 24 hours’ worth of saved breastmilk or formula available in case this occurs. Depending on how much of the allergen you ate and how severe the symptoms, you can decide when you feel comfortable nursing again."
I guess if you only feed him in the morning, you could experiement with eggs for breakfast, as it will then be another 24 hours before you feed him again - but I would start with a very small amount of very well cooked egg to begin with, such as in shop-bought processed pancakes and waffles. More examples of well-cooked egg are in the 'egg ladder': https://www.thh.nhs.uk/documents/_Patients/PatientLeaflets/paediatrics/allergies/PI414_Egg-Ladder-1.pdf
Hopefully soon Dr Trill's breastmilk testing strips will be available so you would be able to know for sure whether a particular allergen is present in your breastmilk.
Kelly
Hi,
First of all, thank you for posting this online! It has been super helpful as my research yielded the same results about it staying in my breast milk for 3 weeks and I was thinking of dumping all my breast milk supply I had saved for 6 weeks while I was eating dairy.
I have been doing an elimination diet for a month to help my exclusively breastfed 3 month old. I know she’s allergic to dairy and just reintroduced soy but she started showing Symptoms. I was wondering if it’s the same with soy protein and if it will take about 2 weeks to clear from her system?
Thanks,
Kelly
Zoe
Hi Kelly, if you only reintroduced soy for a short time after your exclusion diet then then the symptoms should get better much more quickly, as there will be much less in her system. It's very common for little ones who react to milk to also react to soya.
Good luck with your reintroductions! It sounds like you are doing a great job.
Brittney Eckert
I’m seeing the 4/15/20 update to the article. What does this mean? Are the recommendations now 7 days? Are you seeing this in your studies? Is this only some women (outliers) or is this common? I’m so sad to hear 7 days.
Zoe
Hi Brittney, that's a great question - I'm awaiting an comment from Dr Trill regarding this study and will re-update the post very soon. She says that the amount of proteins detected in that study would not be enough to cause a reaction. For this reason it is not a viable study, so you can safely ignore it.
I am not a scientist and just wanted to mention the study for caution, after another doctor brought it to my attention. Sorry for muddying the waters!
Brittney Eckert
Thank you for your response!
Caitriona
Phew! Please can you put this reply in with the update so that it's clearer?
Zoe
I have just updated the update (lol!) - I was just waiting for a full explanation from Dr Trill. As I am not a scientist myself I didn't feel qualified to comment fully until I had heard from her. I hope it is much clearer now.
Shay
I’m beginning to eliminate peanuts from my diet due to coming to the realization that my baby is likely allergic to them. Will frozen breast milk have any different affect on him than when he eats it directly from the breast? I’m worried I’m going to have to restart my frozen stash. What are your thoughts on this?
Zoe
Firstly, if you suspect your baby is allergic to peanuts please seek medical advice! If your baby has a peanut allergy then he will likely need allergy testing, and may need rescue medication in case of a reaction. If he doesn't have an allergy then avoiding peanuts can actually increase the chances of him developing an allergy to them in future. So either way it's really important to get a proper diagnosis.
As for the frozen breastmilk stash, if your breastmilk contains peanut allergens, then freezing it won't make any difference. Has your baby shown any signs of a reaction to breastmilk alone, or has he only reacted when peanuts have been given in solid foods? This is one to discuss with your health professional. You might also want to get on the wait list for Dr Trill's breastmilk testing strips! https://www.freetofeed.com/waitlist
Dianne
Hello Doctor, how long should I wait before I introduce allergens containing food.. I still don't know what triggering the bloody stool of my baby. Currently not eating dairy, gluten, soy, nuts, milk
Zoe
Hi Dianne, sorry to hear you are struggling with your baby's symptoms. I suggest you speak to your doctor for advice.
Lauren
This is a really misleading post and it’s disappointing to see. I hope breastfeeding parents do some more research beyond this. It seems to point towards being able to ‘cheat’ and have some of the eliminated foods, wait 8 hours, and then everything will be okay. In my experience that would result in some very unhappy and poorly babies. This post seems to be based on some shaky evidence from a study looking at a different allergen. Disappointing advice as I said.
Zoe
Hi Lauren, this post is designed to give parents some hope and optimism to continue breastfeeding when they might otherwise give up. I would never encourage parents to deliberately consume their baby's allergen(s) and I'm sure Dr Trill wouldn't either - but I think parents should know that an accidental slip up can be easily managed with a stash of breastmilk in the freezer, and doesn't mean they have poisoned their baby for 2 weeks. We are all only human after all 🙂 This takes away some of the anxiety breastfeeding parents may feel around allergies, which can be a huge issue, as I'm sure you know from your own experiences and the valuable work you do supporting families with CMPA.
If you look further at Dr Trill's website https://www.freetofeed.com/ her assertions are based on more than one study, and a number of different food allergens.
Clare H
Hello I just came across this when desperately googling for some advice regarding my baby who keeps getting blooding stools (and very unhappy) despite eliminating dairy, soya & egg. I was wondering if there are any books or resources I can turn to, to get a better understanding of baby allergies and breastfeeding etc. as it all feels so overwhelming and I want to learn more. I have no medical background so want something in layman’s terms!
Zoe
Hi Clare, I would definitely seek medical advice if your baby has blood in their stools. I have actually written a book myself that guides you through the process, including how to keep a food and symptoms diary to show your doctor, to help them help you! If you subscribe to my newsletter you’ll get the first 3 chapters free, which should get you off to a good start. You can sign up here: https://www.myallergykitchen.com/subscribe-now/
Kristy
my baby is very on happy I ate green beans last night for dinner and today she is crying her tummy hurts and has a lot of air in her tummy and green poop. all day it’s been breast milk poop color now it’s green at 5 o’clock the next night I am scared my milk still has green beans in it and it’s hurting her 🤦🏼♀️ What to do ?
Zoe
Hi Kristy, I'm sorry to hear your baby is having tummy troubles. There are lots of reasons why babies can have green poop, wind and stomach ache. Sometimes it can be due to a food allergy, but there are other possible reasons.
You don't say how old your baby is or whether she has any other food allergies already. If you suspect she might be reacting to something you ate, I would suggest you keep a food and symptoms diary and consult a doctor for further advice.
This post about symptoms of cow's milk protein allergy will give you a basic introduction to food allergy in babies https://www.myallergykitchen.com/does-my-baby-have-cmpa-signs-symptoms-of-cows-milk-protein-allergy/. The symptoms are similar for all foods; milk is the most common allergy in babies.
I hope your baby feels better soon, if her symptoms continue or get worse please speak to a doctor.
Chris Pederson
Wow, I had no idea that food allergens take about 2-8 hours to clear from breast milk. My wife is due any day now. We both have severe allergies to different foods so we'll get out little boy tested for allergens as soon as possible to be safe.
https://www.agrifood.com.au/index.php/services/food-safety
Dani
Hi there! Thank you for your post. My 2 month old has moderate-severe eczema. I think it may be temperature related but I am trying food as well to be safe. I eliminated allergens for one week and reintroduced peanuts for 2 days. No peanuts day 3 or 4. Your post says is should clear in 2-8 hours. Her rash got worse on day 4. Does that seem unrelated to the peanuts or could it still be peanut allergy? Thanks!
Zoe
Hi Dani, sorry to hear that your little one is suffering with eczema. If you suspect food could be the problem you must speak to a doctor for advice. I would suggest keeping a food and symptoms diary - write down everything you eat and any symptoms in your baby. This is excellent evidence to show a health professional and help you identify any patterns. Although peanut allergens would clear from breastmilk within hours, it can take up to 3 days for baby to show signs of a reaction in some cases. Every individual is different, hence why it is best to speak to a doctor.
Michelle C
Why would it take the infant up to 10 days - a few weeks to clear an allergen, and not adults?
Also, the update about the amount of cows milk protein remaining in the milk and how much is needed to be present in milk to cause a reaction- isn’t that problematic to state? If a child is highly allergic, won’t they react regardless?
I’ve seen my LO react every time I’ve tried any of his allergens, let alone him consuming himself.
Zoe
Excellent questions, thank you for your comment Michelle! Before I respond I would like to remind you that I am answering as a mum and not a scientist or doctor. If you need further advice please speak to your health professional or you can contact Dr Trill directly through her website freetofeed.com
It takes time for anyone to clear an allergen 'out of their system'; baby, child or adult. When you eat, the nutrients in the food are used, in part, to regenerate cells in your body - they literally become part of you, part of your skin, hair, nails, organs etc - part of your system. This is an endless, ongoing cycle and so eventually these cells will die and be replaced, so they are 'out of your system'. For example some intestinal cells are replaced every 5 days, so it can take that long to notice an improvement in intestinal symptoms. Skin cells are replaced every 39 days so it may take about a month for eczema to clear.
However, my understanding is that breastmilk is made from whatever you are eating. If you stop eating dairy, there will be no dairy in your breastmilk after a few hours.
I added the update to acknowledge the study about cow's milk protein reminaing in breastmilk for completeness, and because it was creating a misconception that I wanted to address. Science has determined that there is a certain level of dairy protein that will cause a reaction, and below that level it won't cause a reaction. This is similar to the law on labelling foods as 'gluten-free' which states that foods must contain 20ppm or less of gluten - so a food can contain 19ppm of gluten and still be safe for people with coeliac disease. So although the milk protein can be detected, it's only in trace amounts that are not considered to cause an allergic reaction. Of course allergies are individual, which is why you must always seek medical advice for personalised support.
You say that your LO always reacts when you try his allergens, so you are an indivdual for whom those food proteins do travel into your breastmilk. However you don't state the period of time after eating the allergen that you feed him and notice a reaction. You may find that if you had an accidental slip up (e.g. you accidentally ate one portion of food containing the allergen), and fed your son previously pumped milk (e.g. from a freezer stash) for 24 hours, you may be able to safely breastfeeding him again after this time, without a reaction. However I don't suggest that you experiment with this deliberately! You should always avoid your son's allergens unless you are advised otherwise by a medical professional.
This is exactly the reason why Dr Trill has developed her breastmilk testing strips, so that when mums do have a slip up (we are all only human after all) you can know for sure when your breastmilk is clear of your baby's allergens.
Jennifer
So glad I found your post. My 7month old son is having a very bad rash from me..I had an entire crab for dinner last night. It started as a little rash on the belly, over night it went all over his body and face/neck. Should I stop feeding him? Tried giving him formula and he just won’t take it and basically cried himself to sleep everytime. But he’s basically starving, and still won’t take the bottle. So I kept feeding him, but the rash just gets worst.
Zoe
Hi Jennifer, I would suggest you write down everything you ate and any symptoms your son has had. Take a photo of the rash. Take it all to your doctor for advice as soon as possible. Hopefully the rash will fade quickly as the crab gets out of your system, but even if it does your doctor may want to refer your son for allergy testing.
Joyce
Should I do formula while my daughter rash/hives clears up from my milk? Or can I keep feeding her? Her entire body and face has rash/hives but she behaves normal
Zoe
Hi Joyce, a rash/hives can be a sign of an allergic reaction so you should seek medical advice as soon as possible. Keep a diary of what you are eating and any symptoms you see in your daughter, as this may help to figure out what is causing the reaction.
Sarah
Hi! I found this so informative! Our daughter was first diagnosed with a milk allergy at a few weeks old, since the link between milk and soy allergy is so high we eliminate milk and soy in tandem. Later I had a suspicion eggs were part of the issue, threw elimination and reintroducing it was confirmed, i ate 2 eggs for 3 days and on day 4 she had bloody stool. She is now 10 months and doing wonderful.
I would like to know about the soy, do you know or can you point me in the direction of any reliable information on how long soy stays in breastmilk and how long it take to see a reaction?
Thanks for taking your time.
Zoe
Hi Sarah, Thank you for your comment. I'm glad to hear your baby is doing well on a dairy, soya and egg free diet. I would head over to Dr Trill's website Free To Feed and get in touch with her for more detailed information about soy in breastmilk.
Erin
Celebrating my baby’s first birthday with a homemade (free from all the food sensitivities that she and sister have) ice cream cake... only to find her having an immediate rash and hive reactions! So it looks like maybe we have more to eliminate. Sigh. What sort of allergy and/or sensitivity testing would you recommend? With our toddler the allergy testing showed no real allergies(except pineapple) but she CLEARLY has immediate reactions to certain foods, which are now eliminated. Both girls suffer from constant eczema on their hands (and occasionally elsewhere).
We are currently free of: dairy, egg, soy, gluten, tomato, cod, pineapple, avocado (for infant), and now potentially cashews (trying to figure out the birthday cake incident). Any advice would be very greatly appreciated. Also, infant has unexplained Failure to Thrive (drs do NOT think it is calorie intake related).
Zoe
Hi Erin, sorry to hear of your allergy struggles. I would definitely ask your child's doctor about allergy testing. Look carefully at the ingredients in the ice cream cake, and anything else she ate that day that was out of the ordinary, as I'm sure you already have.
Home allergy/intolerance/sensitivity testing is generally NOT reliable so don't waste your money.
Lauren
This is the most helpful thing on the internet. Thank you!!!
Devin
Hi! I have been trying to be dairy free for just under 2 weeks for my 2 month old. What if I accidentally ate a product that contains milk and fed him without realizing it? That was about 3 days ago. He was doing so much better with his symptoms after about a week of me be dairy free. But now he seems gassy. How long would you expect that one slip up to affect him? Is it reasonable to think that his gassiness now could be due to a slip up from 3 days ago? Thank you!
Zoe
Hi Devin, you might like to check out this blog post which covers symptoms and timing of allergic reactions in more detail https://www.myallergykitchen.com/introduction-to-cows-milk-protein-allergy/
He should recover more quickly from a small slip up.