MCAS and histamine intolerance

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Summary of histamine-mediated symptoms from

MCAS stands for mast cell activation syndrome. Some MCAS specialists believe that overactive mast cells are responsible for a wide range of health problems; they may even be responsible for long COVID. While MCAS may be difficult to diagnose through medical testing, some patients are able to reduce their symptoms through a trial and error process of trying different foods and medications.

Histamine is a chemical normally produced by your body that is released into the bloodstream when your immune system is defending against a potential allergen. The theory is that some people are prone to accumulating too much histamine in their body, which can lead to toxic effects. MCAS and histamine intolerance are related because they have overlapping symptoms and treatments.

Some long haulers report that a low histamine diet and/or MCAS treatments improve their symptoms.

Low histamine diet[edit]

Histamines can cause reactions that mimic allergic reactions such as:

  • Itchiness
  • Redness with flushing
  • Rash/eczema
  • Hives / urticaria (red, itchy welts)
  • Dermatographia (you can 'draw' on your skin)
  • Rosacea (blushing or flushing and visible blood vessels in your face)
  • Psoriasis (red, itchy scaly patches)

Histamines can also cause symptoms that are common in long haulers: brain fog, insomnia, anxiety, fatigue, etc.

According to Tina Peers, histamine intolerance should be diagnosed if the patient has two or more symptoms and see improvement when eating a low-histamine diet for 4 weeks while taking antihistamines. Because the diet can be tricky to implement in the beginning, she suggests consulting a nutritionist or dietician familiar with the condition.

A low histamine food list can be found here:

For those with MCAS, the same group (SIGHI) has compiled a more detailed (and complicated) food compatibility list:

Scientists may be wrong about how low histamine diets work[edit]

Firstly, there is no scientific consensus as to what foods are low or high histamine. Table 1 in a paper by Martin, Brachero, and Vilar summarizes different claims in the scientific literature about a wide range of specific foods. You should not be surprised if online food lists contain conflicting information about low versus high histamine. In practice, patients can pick a single food list and experiment to figure out what works and what doesn't.

Secondly, the gut contains histamine-producing bacteria and food that they live on. For a deep dive into the scientific evidence 'supporting' histamine intolerance, see the paper by Hrubisko and his colleagues titled Histamine Intolerance—The More We Know the Less We Know. A Review.

While the theory supporting low histamine diets may be wrong, some people seem to see an improvement in their symptoms from following a low histamine diet. While we may not be certain about how it works, it is a path of experimentation that may improve symptoms. Patients may discover that their food intolerances do not necessarily perfectly line up with the theoretical content of histamines, histamine liberators, DAO (enzymes that break down histamines), etc. It is also possible for patients to have non-histamine food intolerances such as gluten sensitivity (including the non-celiac version of gluten sensitivity that returns a negative celiac biopsy test).


Antihistamine drugs do not lower histamine levels but they block the receptors for histamines. "Blocker" drugs can block histamines from attaching to H1 and H2 receptors. Both H1 and H2 can be blocked to reduce symptoms.

Over-the-counter antihistamines include:

  • Loratadine (Claritin), fexofenadine (Allegra), cetirizine (Zyrtec, Benadryl Allergy One a Day Relief 🇬🇧), levocetirizine (Xyzal) and acrivastine (Benadryl Allergy Relief 🇬🇧) are H1 blockers that are sold over-the-counter in most countries. Some of the drugs require a prescription in some countries, e.g. fexofenadine in the UK.
  • Famotidine (Pepcid) is a H2 blocker

Some patients find that not all H1/H2 blockers improve their symptoms so trial and error is needed to find a blocker that they respond to.

DAO enzymes taken 30-60 minutes before a meal may be useful as DAO enzymes break down histamines. They may come in handy for social situations which involve eating foods that might have histamines in it.

Many other drugs (mostly prescription) are used to treat MCAS. Examples of such drugs include:

  • Other H1 and H2 blockers
  • Mast cell stabilizers
  • Benzodiazepines. These are controlled substances that can lead to serious drug dependency/withdrawal problems if used for long periods of time. Your doctor may not necessarily provide you with informed consent regarding the difficulties some patients face trying to get off these drugs. Because these are controlled substances, refilling a prescription on short notice can be difficult so you should be diligent in refilling your prescriptions to avoid accidental withdrawal.
  • Low dose naltrexone (LDN)

MCAS specialists and doctors who educate themselves regarding MCAS (e.g. to treat long haul patients) would be able to guide you through such drugs and be able to provide you with a prescription.

MCAS specialists[edit]

The patient support organization The Mast Cell Disease Society has a list of medical centers treating MCAS at

MCAS specialists can also perform various medical tests to diagnose MCAS. However:

  • The tests often return false positives, e.g. when the patient isn't in the middle of a flare.
  • The tests for MCAS do not predict whether or not a particular treatment will alleviate symptoms.

Guides and resources for HIT and MCAS[edit]

MCAS resources and introductory guide written by a MCAS sufferer at

Tina Peers has a histamine intolerance factsheet

Food lists:

DrBeen and other Youtubers have online discussions with MCAS specialists and/or discussions about MCAS in long haulers.

A MCAS diagnosis and treatment guideline by Lawrence Afrin and his colleagues can be found at