MCAS and Medication Excipients: When the Reaction Is Not the Drug

💊 The prescription is the same. The dose is the same. The pharmacy handed you a different box this month, and within a week you are worse, and nobody can explain why.

🔍 A medicine is mostly not medicine. Around the active drug sit dyes, binders, coatings, preservatives, sweeteners and anti-caking agents, collectively called excipients, and officially described as inactive. For most people they are. In mast cell activation syndrome they are a documented source of reactions that get recorded as intolerance to the drug itself.

💚 This matters because the conclusion drawn is usually the wrong one. A medicine that might have helped you goes on the allergy list, permanently, when the problem was a colourant that a different manufacturer does not use.

🏷️ Why the label does not answer your question

Regulators do require some excipients to be declared, but only a specific list of them. New Zealand's medicines regulator publishes its version under the heading "Excipient(s) with known effect". Lactose is on it, because of rare hereditary problems of galactose intolerance, total lactase deficiency and glucose-galactose malabsorption. Gluten-containing ingredients are on it too, and the New Zealand rule is to declare wheat starch and any other gluten-containing ingredient at three parts per million or more.

That list exists to flag a small number of known population-level problems. It was never designed to tell a mast cell patient what is in their tablet. So the absence of a warning on a box tells you very little, and the presence of a full ingredient list is something you have to go and find.

🧪 Which excipients come up, and which rarely do

The most useful clinical account of this comes from Schofield and Afrin, writing in the American Journal of the Medical Sciences in 2019, who set out a series of patients to illustrate the pattern. Their starting advice is specific: when excipient reactivity is suspected, dyes and alcohols are good places to start.

Identified in their actual case reports, which is the strongest evidence the paper offers:

• FD&C dyes. One patient's nortriptyline capsule contained five different FD&C dyes, alongside benzyl alcohol. The paper describes that patient, an eighteen year old man, as extremely sensitive to alcohol, with acute onset of symptoms on minimal ingestion. In the authors' experience, red dyes are the most poorly tolerated, followed closely by yellow and then blue.

• Benzyl alcohol. In that same formulation, and alcohol in a trigger point injection that made another patient's pain acutely worse and also caused hives and flushing. The authors note that all intravenous formulations of lorazepam available in the United States at the time of writing are stabilised with alcohol.

• Sodium lauryl sulfate. Implicated in two of their patients, once in a refill from a different manufacturer that produced a decline over several weeks.

• Crospovidone, talc and aspartame, through one patient's orally disintegrating clonazepam.

Stated more generally by the paper, outside those cases, and therefore a weaker claim worth treating as one. It describes alcohols as common triggers in MCAS patients, naming benzyl alcohol and polyvinyl alcohol. It separately lists magnesium stearate, polyethylene glycol, shellac, povidone and sodium hydroxide among excipients that have caused problematic reactivity in some MCAS patients. And it notes that many MCAS patients react to the preservatives in multidose products, the kind found in nasal sprays and eye drops.

Equally useful is what the authors describe as usually tolerated. Ferric oxide red and yellow are colourants that tend to be well tolerated by most MCAS patients, which means a coloured pill is not automatically a problem. Microcrystalline cellulose and gelatin are generally very well tolerated, although some people do react.

And one detail that reads like a curiosity until it is your week: one patient reacted to cotton-derived microcrystalline cellulose in a compounded medicine and tolerated the wood-derived version of the same excipient.

🔎 The signal that tells you it is the excipient

There is one pattern worth knowing by heart. Excipient reactivity should be suspected if different responses are seen to different formulations of the same drug. Same molecule, same dose, different box, different outcome.

Three variations on that, all from the same paper, that explain a lot of apparently random months:

• Different strengths of a medication made by the same manufacturer may contain different excipients.

• A patient may tolerate an oral formulation and not the injectable one, or the reverse.

• Two brands containing only the active drug and sterile water can still behave differently.

The authors also warn that it is not always dramatic. Some excipient reactions are obvious, while others cause a more insidious decline, which is why a change of manufacturer is worth checking for when things drift without explanation.

🖥️ How to find out what is actually in it

In the United States the answer is a public database. The authors recommend reviewing medications on the DailyMed website before picking them up from the pharmacy, and describe it as an invaluable resource for patients, physicians and pharmacists alike. Parenteral products and their excipients are in there too.

They are blunt about where the work sits: manual review of ingredient lists should be done by both pharmacists and patients, and patients benefit from taking ownership of knowing which excipients are present.

Two practical habits follow. Keep a record of the products you know you tolerate, manufacturer included, which is the same discipline as what belongs on a medication list. And remember that supplements carry excipients exactly as prescriptions do, which is worth holding alongside our overview of supplements in MCAS and POTS.

🗣️ What to do with it, and what not to do alone

The single most useful change the paper suggests is in how a reaction gets recorded. When an excipient is identified, it should be added to the allergy list rather than the medication being listed. That one distinction keeps a drug available to you for the rest of your life.

Beyond that, the levers belong to your prescriber and your pharmacist. A different manufacturer of the same drug. A different strength or formulation. For severely reactive patients, the authors describe compounding pharmacies as invaluable, with some people needing preparation in an ultraclean compounding facility.

Two things not to do on your own. Do not stop or change a prescribed medicine because you suspect an excipient: take the suspicion to the person who prescribed it, with your record. And although the paper notes that some but not all capsule-contained medications can be opened and their contents taken without the shell, that is not safe for every product, because some capsules and coatings control how and where a drug is released. Ask the pharmacist about your specific medicine first.

Triggers outside the medicine cabinet still matter, of course, and our guide to common MCAS triggers covers those. The authors add one crossover worth noting: shampoos and toothpastes may contain dyes too.

❓ Frequently asked questions

📚 Sources & further reading

The information in this article is drawn from the following sources. We encourage you to explore them.

• Schofield JR, Afrin LB: Recognition and Management of Medication Excipient Reactivity in Patients With Mast Cell Activation Syndrome. Am J Med Sci. 2019;357(6).

• Mast Cell Action: Recognition and management of excipient reactivity in MCAS (full paper)

• Medsafe New Zealand: Declaring excipients with known effect