The Claritin and Pepcid “Immune Protocol” for IVF: What It Actually Is, Who It’s For, and Why Most Clinics Are Misusing It
A reproductive immunologist on the antihistamine protocol now spreading through major IVF centers — and what should happen before a patient is ever placed on it
An editorial by Andrea Vidali, MD Chief Medical officer and Founder ESSI; Co-founder, The Endometriosis Summit; CEO, Pregmune
Summary: Claritin (loratadine) and Pepcid (famotidine) are H1 and H2 histamine-receptor blockers, not a comprehensive “immune protocol.” The combination has a legitimate, mechanism-based role in IVF patients with documented mast-cell or IgE-driven pathology — including many with endometriosis or adenomyosis — but it is now being dispensed empirically at major IVF centers to almost any patient with a failed transfer, without an IgE level, an atopy history, or an evaluation for endometriosis. That is not personalized medicine; it is appeasement. Patients deserve a workup before a protocol.
A protocol I helped invent is now being misused
More than fifteen years ago, working alongside the late Dr. Jeffrey Braverman at Braverman Reproductive Immunology, we began using a combination of H1 and H2 receptor blockers — typically loratadine (Claritin) and famotidine (Pepcid), occasionally with diphenhydramine (Benadryl) or low-dose prednisone — in a very specific subset of patients with implantation failure and recurrent pregnancy loss. We wrote about it openly on our website. We discussed it at conferences. Patients found it. Forums talked about it. And eventually it spread.
Today, that same combination is being marketed at major IVF centers — most prominently a large Colorado-based program whose initials patients in the IVF community will recognize without needing to be told — as an “immune protocol” and dispensed broadly to almost any patient who has had a failed transfer or a miscarriage and asks for “something immune.“
The original Braverman / BRI writeups on antihistamine use in IVF and recurrent pregnancy loss are still publicly archived and predate the current vogue by more than a decade. [1] [2]
I have a problem with what is happening now, and I want to be specific about why.
Problem #1: It is not an immune protocol
Calling Claritin and Pepcid an “immune treatment” is, frankly, marketing. These drugs are receptor antagonists. Loratadine blocks the H1 histamine receptor; famotidine blocks H2. [3] [4] They blunt the downstream effects of histamine that has already been released, primarily by mast cells.
They do not modulate T-cell tolerance, NK cell function, cytokine balance, complement activation, antibody production, or any of the other adaptive immune machinery that governs implantation and the maintenance of pregnancy. This matters because reproductive immunology is a real, sophisticated, and rapidly maturing discipline — and it deserves to be represented honestly.
Lumping two over-the-counter receptor blockers together with the broader toolkit a reproductive immunologist actually uses, and calling the result an “immune protocol,” is a category error. H1/H2 blockade belongs to a much narrower conversation: mast-cell and histamine biology. That is one slice of reproductive immunology, not a synonym for it.
The visual below makes the distinction explicit:
In the broader allergy and mast-cell literature, dual H1/H2 blockade is exactly what it sounds like — symptomatic management of histamine-driven disease such as chronic urticaria, mastocytosis, and mast cell activation syndrome. [5] [6] H1 antagonists like desloratadine do have some additional mast-cell-stabilizing properties at higher concentrations, [7] which is interesting, but it does not transform a receptor blocker into a comprehensive immune modulator.
Problem #2: The biological rationale is real — but narrow
This part is important, because I do not want to throw out the baby with the bathwater.
Mast cells are present throughout the endometrium across the menstrual cycle and become activated around the time of implantation. [8] [9] Their mediators — histamine, tryptase, TNF-α, VEGF — participate in trophoblast invasion, angiogenesis, decidualization, and spiral artery remodeling. [9] [10] Histamine itself contributes constructively to blastocyst implantation and placental development. [9]
Mast-cell density is dramatically increased in endometriotic lesions, [11] [12] in adenomyotic uteri, and in endometrial polyps, [13] and is modulated by estrogen. [14] In patients with mastocytosis or mast cell activation syndrome, pregnancy is a recognized trigger for mediator release with real obstetric consequences. [9] [10]
So yes — in a patient whose implantation environment is being disrupted by an IgE-driven or mast-cell-driven inflammatory process, antagonizing H1 and H2 receptors is mechanistically rational. That is the patient population this protocol was designed for: women with documented atopy, elevated IgE, or an underlying endometriotic / mast-cell-rich phenotype. It was always meant as a targeted intervention based on the individual patient’s biology, not a generic IVF add-on.
Problem #3: There is essentially no evidence for using this in everyone
Let me be precise. After more than a decade of clinical use across multiple centers, there is still:
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No randomized controlled trial demonstrating that empiric, untargeted H1/H2 blockade improves live birth rates, implantation rates, or reduces miscarriage in unselected patients with implantation failure or recurrent pregnancy loss.
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No prospective controlled efficacy data for the so-called “Colorado protocol,” “CPP protocol,” or any of its variants.
The available human literature on antihistamines around the time of pregnancy is almost entirely safety data — and it is reassuring, [15] [16] [17] [18] [19] but safety is not the same thing as benefit. One center publishing its own patient handout on the “Claritin/Pepcid/Prednisone” protocol states this directly: “The CPP protocol has not been validated in peer reviewed studies.” [20]
There is also a retrospective signal worth being honest about. An 8,873-cycle frozen embryo transfer cohort study found that, among normally ovulating patients, the untargeted, empiric use of adjuvants in cycles where they were not specifically indicated was associated with worse outcomes than routine treatment. [21] One observational study does not settle a question, but it should make us pause before adding any medication reflexively, without an indication.
Problem #4: The hypocrisy
Here is what bothers me most. When a patient walks into one of these clinics after three failed euploid transfers and demands “something immune,” the clinic does not measure her IgE. It does not take an atopy history. It does not look for endometriosis or adenomyosis. It does not check for mast-cell activation.
It hands her two over-the-counter pills and calls it a protocol — because the drugs are cheap, the drugs are safe enough, and the drugs make the patient feel like something is being done. That is not personalized medicine. That is appeasement. And appeasement medicine has its own cost: it substitutes a slogan for a workup, it delays a real diagnosis, and it fails the very patients the protocol was originally designed to help.
Problem #5: Histamine is not uniformly the enemy
There is a deeper conceptual problem with empiric pan-blockade of H1 and H2 at the implantation window. As discussed above, histamine is part of the normal implantation choreography — it contributes to trophoblast invasion, vascular permeability, and angiogenesis at the maternal–fetal interface. [9] [10]
In a patient with no evidence of histamine-driven pathology, what exactly are you accomplishing by blocking it? At best, nothing. At worst, you are flattening a signal that the embryo is using. This is not theoretical hand-wringing. It is the same principle that applies to every other class of receptor-targeted drug in medicine: blockers work when the system is overactivated. When it isn’t, you get side effects without benefit, and sometimes you get the opposite of what you wanted.
What real reproductive immunology looks like before this protocol
Before any patient is placed on H1/H2 blockade for implantation failure or recurrent pregnancy loss, she deserves a workup that actually tells you whether the protocol could plausibly help her. A reproductive immunologist’s approach is the opposite of one-size-fits-all.
The minimum workup before this protocol should include:
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Serum total IgE: The cheapest, easiest screen for an atopic / mast-cell-driven phenotype. Elevated IgE in unexplained implantation failure is a real signal worth acting on.
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Atopy and allergy history: Allergic rhinitis, asthma, eczema, food allergies, urticaria, hives during prior pregnancies, flushing or rashes after embryo transfer, reactions to progesterone-in-oil. Often more informative than any single lab.
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Evaluation for endometriosis and adenomyosis: Both are mast-cell-rich pathologies of the reproductive tract and are massively underdiagnosed in the IVF population. A patient with undiagnosed endo or adeno is far more likely to benefit from mast-cell-directed therapy — and from treating the underlying disease.
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Serum tryptase (selected patients): When mastocytosis or mast cell activation syndrome is on the differential: reproducible flushing, GI symptoms, urticaria, anaphylactoid reactions, or unexplained reactions during prior cycles.
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Comprehensive reproductive-immunology panel: NK cell number and activity, cytokine profile, HLA work, autoantibodies, thrombophilia. This is where reproductive immunology actually lives — and it is the rest of the conversation that the antihistamine protocol cannot substitute for.
If the workup is positive for a mast-cell or IgE-driven phenotype, H1/H2 blockade is a reasonable component of a broader, individualized plan that also addresses the underlying disease. If the workup is negative, the patient does not need Claritin and Pepcid. She needs a different conversation.
The Bottom Line
The H1/H2 antihistamine protocol is a legitimate, mechanistically coherent intervention — for the right patient. It is not, and never was, a universal IVF add-on, and the way it is currently being marketed bears almost no resemblance to the targeted intervention Dr. Braverman and I described over fifteen years ago.
Reproductive immunology done well is patient-specific, mechanism-driven, and accountable to the underlying biology. Handing every failed-transfer patient the same two over-the-counter drugs under an “immune protocol” label is the opposite of all three. It is convenient for the clinic, emotionally satisfying for the patient, and intellectually lazy.
Patients deserve better. They deserve a workup before a protocol. They deserve a reason for every drug they are asked to take. And they deserve clinicians who know the difference between blocking a histamine receptor and actually evaluating a reproductive-immunologic problem.
If you are a patient who has been offered “the antihistamine protocol” without an IgE level, an atopy history, or an evaluation for endometriosis — ask why. “Because everyone is doing it” is not an answer.
Frequently Asked Questions about the Claritin and Pepcid IVF Protocol
What is the “antihistamine protocol” or “CPP protocol” in IVF? The antihistamine protocol is a regimen built around two over-the-counter histamine-receptor blockers — loratadine (Claritin), an H1 antagonist, and famotidine (Pepcid), an H2 antagonist — sometimes combined with low-dose prednisone. The combination is also referred to as the “CPP protocol” (Claritin / Pepcid / Prednisone) or as a “Colorado protocol” because of where it has been most prominently marketed. It is most often given around the time of embryo transfer and into early pregnancy, in patients with implantation failure or recurrent pregnancy loss.
Are Claritin and Pepcid actually an “immune protocol”? No, not in the way that phrase is usually understood. Claritin and Pepcid are histamine-receptor antagonists; they block the downstream effects of histamine that has already been released, primarily by mast cells. They do not modulate T-cell tolerance, NK cell activity, cytokine balance, complement, or antibody production. Calling them an “immune protocol” lumps them in with categorically different drugs (prednisone, IVIG, intralipids, G-CSF) and obscures what they actually do.
Who is the antihistamine protocol actually appropriate for? Patients in whom there is real evidence of mast-cell or IgE-driven involvement in implantation failure or pregnancy loss. That typically includes patients with elevated total serum IgE, a strong atopic history (allergic rhinitis, asthma, eczema, urticaria), endometriosis or adenomyosis (which are mast-cell-rich pathologies), and patients with mastocytosis or mast cell activation syndrome (MCAS).
What workup should happen before a patient is placed on the antihistamine protocol? At minimum: a serum total IgE, a careful atopy and allergy history, a real evaluation for endometriosis and adenomyosis, and serum tryptase in patients in whom mastocytosis or MCAS is on the differential. A broader reproductive-immunology workup (NK cells, cytokine profile, HLA, autoantibodies, thrombophilia) is appropriate in patients with unexplained recurrent loss or implantation failure. If the workup is positive, the protocol is reasonable as part of a broader plan. If it is negative, the patient does not need this protocol.
Is there randomized controlled trial evidence that Claritin and Pepcid improve IVF outcomes? No. There is no randomized controlled trial demonstrating that empiric H1/H2 blockade improves live birth rates, implantation rates, or reduces miscarriage in unselected IVF patients. The available human data on antihistamines around the time of pregnancy are primarily safety data, which are reassuring, but safety is not the same thing as benefit. At least one clinic that prescribes the protocol acknowledges in its own patient handout that it has not been validated in peer-reviewed studies. [20]
Are Claritin and Pepcid safe in pregnancy? The available data — large prospective cohorts and meta-analyses — are reassuring for both loratadine and famotidine in the first trimester and beyond. [15] [16] [17] [18] [19] Safety in pregnancy is not the question being raised here; the question is whether they should be prescribed empirically, without an indication, to patients who do not have evidence of histamine-driven pathology.
What’s the harm in taking Claritin and Pepcid “just in case”? Two things. First, in a patient with no histamine-driven pathology, you may be flattening a signal that the implantation process actually uses — histamine plays constructive roles in trophoblast invasion, vascular permeability, and spiral artery remodeling. Second, and more importantly, prescribing the protocol empirically often substitutes for the workup that would tell you what is actually wrong. Patients with undiagnosed endometriosis, adenomyosis, or true reproductive-immunologic disease end up on a generic regimen that delays the real diagnosis.
Why do major IVF clinics prescribe this if there’s no RCT supporting it? Because the drugs are cheap, well-tolerated, and emotionally satisfying for patients who have been through multiple failed transfers and want “something immune” to be done. The protocol functions as a low-risk gesture rather than a targeted intervention. That is what we mean by appeasement medicine.
A note on history
The original BRI / Braverman writeups on antihistamines in IVF and recurrent pregnancy loss remain publicly archived and predate the current widespread adoption by more than a decade. [1] [2] The framework Dr. Braverman and I developed always tied this protocol to a documented immunologic indication — most often elevated IgE, atopic disease, or endometriosis-associated inflammation. It was never intended as a universal add-on, and the way it is being marketed today bears little resemblance to the targeted intervention we described.
References
[1] Braverman Reproductive Immunology. Antihistamines (Claritin) to Prevent IVF Failure.
[2] Braverman Reproductive Immunology. Antihistamine Protocol or Wobenzym.
[3] Patel P, Akhondi H. Loratadine. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025.
[4] UK Teratology Information Service. Use of H2 Receptor Antagonists in Pregnancy.
[5] The Mast Cell Disease Society. Medications to Treat Mast Cell Diseases.
[6] Allergy & Asthma Network. How are Mast Cell Diseases Treated? (Podcast with Dr. Joshua Milner.)
[7] Levi-Schaffer F, Eliashar R. Mast cell stabilizing properties of antihistamines. J Invest Dermatol. 2009;129(11):2549–2551. [8] Elieh Ali Komi D, Shafaghat F, Haidl G. Significance of mast cells in spermatogenesis, implantation, pregnancy, and abortion. Am J Reprod Immunol. 2020;83(5):e13228.
[9] Woidacki K, Zenclussen AC, Siebenhaar F. Mast cell-mediated and associated disorders in pregnancy. Front Immunol. 2014;5:231.
[10] Ferrari J, Benvenuti P, Bono E, Fiorelli N, Elena C. Mastocytosis: fertility and pregnancy management in a rare disease. Front Oncol. 2022;12:874178.
[11] Makoui MH, et al. The role of mast cells in the development and advancement of endometriosis. Am J Reprod Immunol. 2025.
[12] Hart DA. Targeting mast cells as a viable therapeutic option in endometriosis. EMJ Reproductive Health. 2017;3(1):76–83. [13] Al-Jefout M, Black K, Schulke L, et al. Novel finding of high density of activated mast cells in endometrial polyps. Fertil Steril. 2009;92(3):1104–1106.
[14] McCallion A, Nasirzadeh Y, Lingegowda H, et al. Estrogen mediates inflammatory role of mast cells in endometriosis pathophysiology. Front Immunol. 2022;13:961599.
[15] Etwel F, Faught LH, Rieder MJ, Koren G. The risk of adverse pregnancy outcome after first trimester exposure to H1 antihistamines: a systematic review and meta-analysis. Drug Saf. 2017;40(2):121–132.
[16] Moretti ME, Caprara D, Coutinho CJ, et al. Fetal safety of loratadine use in the first trimester of pregnancy. J Allergy Clin Immunol. 2003;111(3):479–483.
[17] Diav-Citrin O, Shechtman S, Aharonovich A, et al. Pregnancy outcome after gestational exposure to loratadine or antihistamines. J Allergy Clin Immunol. 2003;111(6):1239–1243.
[18] Gill SK, O’Brien L, Koren G. The safety of histamine 2 (H2) blockers in pregnancy: a meta-analysis. Dig Dis Sci. 2009;54(9):1835–1838.
[19] Nishimura A, Furugen A, Kobayashi M, et al. Effects of famotidine use during pregnancy: an observational cohort study. J Pharm Health Care Sci. 2024;10:70.
[20] Pollin Fertility. CPP Protocol — patient handout.
[21] Fan Y, Wang Y, Luo Z, et al. Effects of anticoagulants and immune agents on pregnancy outcomes and offspring safety in frozen-thawed embryo transfer cycles (8,873 cycles). Front Endocrinol. 2022;13:884972.
