Sources & method

This site does three things: it reads a product's inactive ingredients from its official FDA label, compares them against a short list of ingredients with published reports of reactions, and finds other manufacturers of the same drug. Here is exactly what we use and how.

How we rate evidence

• Commonly reportedAmong the excipients most often reported to cause drug hypersensitivity reactions, including anaphylaxis.
• Rarely reportedPublished reports of hypersensitivity reactions exist, but they are uncommon.
• Limited evidenceFrequently raised by patients or plausible, but published evidence of reactions is thin or unclear.
• PotentialLittle or no direct evidence, but related to a known culprit, reported anecdotally, or in a category MCAS clinicians and patient groups warn about.
• For informationOften asked about. Current evidence suggests it is rarely a problem.

Ratings reflect published evidence about excipient hypersensitivity in general. Very little research looks at excipients in MCAS specifically, so how often each ingredient causes problems for people with MCAS is not known. Alternatives are ranked by adding up flags (commonly reported = 3, rarely reported = 2, limited evidence = 1). Potential triggers are shown in their own section and only break ties between otherwise equal products.

Inactive ingredients we flag

Active ingredients we flag

These follow Table 12 of Molderings et al. (2016), a published list of drugs with a “proven or theoretical high risk” of mast cell mediator release, with the therapeutic alternatives it suggests. Other studies show some of these drugs activate mast cells directly, without an allergy. Data on how often they cause reactions are limited, and the role of drugs may be overestimated compared with physical triggers.

Research cited

• Pharmacological treatment options for mast cell activation diseaseMolderings GJ, Haenisch B, Brettner S, et al. Naunyn Schmiedebergs Arch Pharmacol 389(7):671-694 (2016).Used for: Table 12, "drugs associated with a high risk of release of mediators from mast cells and their therapeutic alternatives", which the active-ingredient flags follow; and the observation that people with mast cell activation disease often react to excipients (fillers, binders, dyes, preservatives), sometimes very specifically.
• Drug hypersensitivity in clonal mast cell disorders: ENDA/EAACI position paperBonadonna P, Pagani M, Aberer W, et al. Allergy 70(7):755-763 (2015).Used for: That NSAIDs, opioids and perioperative drugs have occasionally caused severe reactions in mastocytosis, while overall data on how often this happens are limited.
• Identification of a mast-cell-specific receptor crucial for pseudo-allergic drug reactionsMcNeil BD, Pundir P, Meeker S, et al. Nature 519(7542):237-241 (2015).Used for: That neuromuscular blockers, fluoroquinolone antibiotics and several peptide drugs activate mast cells directly through the MRGPRX2 receptor, without an allergy.
• Signs, Symptoms & TriggersThe Mast Cell Disease Society (TMS) tmsforacure.org (2026).Used for: Radiocontrast media as a cause of anaphylaxis in some patients, and that triggers differ from person to person.
• MCAS TriggersMast Cell Action mastcellaction.org (2026).Used for: Fragrances and medications as common categories of MCAS triggers.
• Hypersensitivity to Excipients in Drugs: An EAACI Position PaperGarvey LH, Brockow K, Barbaud A, et al. Allergy 81(7):2250-2275 (2026).Used for: Which excipients are most commonly (PEG and related, gelatin, methylcelluloses, povidone, mannitol) and rarely (benzyl alcohol, EDTA, metacresol, metabisulfite, parabens) reported as causes of drug hypersensitivity, and where earlier concerns (lactose) have been largely ruled out.
• Hidden Dangers: Recognizing Excipients as Potential Causes of Drug and Vaccine Hypersensitivity ReactionsCaballero ML, Krantz MS, Quirce S, Phillips EJ, Stone CA Jr. J Allergy Clin Immunol Pract 9(8):2968-2982 (2021).Used for: Gelatin, carboxymethylcellulose, PEG and PEG-related products in immediate reactions; propylene glycol in delayed reactions.
• Anaphylaxis to drug excipientsPfützner W. Allergo J Int 31(5):137-140 (2022).Used for: Examples of anaphylaxis to individual excipients, including dyes such as patent blue.
• Recognition and Management of Medication Excipient Reactivity in Patients With Mast Cell Activation SyndromeSchofield JR, Afrin LB. Am J Med Sci 357(6):507-511 (2019).Used for: That people with MCAS can be more sensitive than average to the inactive ingredients in medications, and that switching formulations can help.
• Management around invasive procedures in mastocytosis: An updateHermans MAW, Arends NJT, Gerth van Wijk R, et al. Ann Allergy Asthma Immunol 119(4):304-309 (2017).Used for: Context that the role of drugs as triggers in mast cell disease may be overestimated, and physical stimuli matter at least as much.

Data

• openFDA NDC Directory: Looking up NDCs and finding other manufacturers of the same drug, strength and form.
• DailyMed (National Library of Medicine): The inactive ingredients listed in each product’s official FDA label.

Lookups are refreshed from these sources at least once a day. openFDA states that its data should not be relied on for medical decisions without verification. Always check the package insert you received.

MCAS organizations

• The Mastocytosis Society (TMS): Patient guides, emergency protocols and trigger information.
• Mast Cell Action (UK): Information on MCAS triggers and self-management.
• MastAttack: Guidance documents and provider primers.
• Swiss Interest Group Histamine Intolerance (SIGHI): Food compatibility list for histamine intolerance.