Mast Cell Activation Syndrome (MCAS)
Mast cells overreacting: flushing, hives, swelling, wheeze, faintness or tummy cramps, in two or more body systems at once. Real but uncommon, and easy to over-diagnose. Here’s what helps.
The MCAS ladder
Most of the evidence is small studies, so treatment is try, review and adjust, ideally with an immunologist. Give each step 2 to 4 weeks before judging it.
- Step 1 · Self-careTrigger diary and a tryptase planSpot the patterns, and get the one blood test that actually confirms MCAS.
- Works
- Not a treatment, so no effect to measure; it’s how the diagnosis gets made (or ruled out)
- Cost
- Diary: free. Tryptase blood test: usually little or no cost with a GP request form
- How long
- Blood 30 minutes to 2 hours after an episode starts, plus a baseline 24+ hours after it settles
- Watch for
- Common triggers: heat, exercise, alcohol, stress, infections, stings, anti-inflammatories, opioids.
Keep the request form in your bag. Episodes never book ahead.
- Step 2 · PharmacyDaily non-sedating antihistamineCetirizine, fexofenadine, loratadine or desloratadine. Blocks histamine, the main chemical behind flushing, itch and hives. The first thing to try.
- Works
- Evidence weak (small studies), but helps many people, especially skin symptoms
- Cost
- About $10–25 for 70 tablets (generic)
- How long
- Every day, not just on bad days. Judge after 2–4 weeks
- Watch for
- Mild drowsiness in some. A specialist may advise up to 4 times the usual dose: don’t freelance that.
Daily means daily. Antihistamines work better as a fence than an ambulance.
- Step 3 · PharmacyFamotidineAn acid-lowering tablet that blocks a second type of histamine receptor. Added for flushing or gut symptoms.
- Works
- Evidence weak (small studies); helps some people, mainly gut symptoms
- Cost
- About $10–20 a month
- How long
- 2–4 weeks to judge
- Watch for
- Usually well tolerated. Occasional headache, constipation or diarrhoea.
Two antihistamines, two different jobs. Teamwork, not doubling up.
- Step 4 · PrescriptionAdrenaline autoinjector, if you’ve had anaphylaxisComes with an ASCIA Action Plan and training. If it’s prescribed, it’s not optional.
- Works
- No dummy treatment trials (they’d be unethical), but the only treatment that reverses anaphylaxis; first-line in every guideline
- Cost
- $25 per script on the PBS ($7.70 concession) for people at risk of anaphylaxis
- How long
- Carry it always. Check the expiry date
- Watch for
- Shaky, pale and racing heart afterwards is normal. Call 000 every time you use it.
The best autoinjector is the one in your bag, not the one in the kitchen drawer.
- Step 5 · PrescriptionMontelukastA tablet that blocks leukotrienes, another mast cell chemical. May help wheeze or flushing.
- Works
- Evidence weak (case reports, small studies); helps some people
- Cost
- About $20–40 a month on a private script
- How long
- About 4 weeks to judge
- Watch for
- Mood or sleep changes, vivid dreams. Stop and tell your doctor if you notice them.
Worth a fair trial. Not worth the nightmares.
- Step 6 · SpecialistMast cell stabilisers or omalizumabSodium cromoglicate by mouth before meals for gut symptoms, or omalizumab injections for severe, repeated anaphylaxis despite other treatment.
- Works
- Small trials, mostly in related mast cell disorders; helps some people
- Cost
- Specialist fees. Oral cromoglicate isn’t routinely available in Australia and can be costly; omalizumab is PBS-subsidised only for certain conditions
- How long
- Cromoglicate: 4–8 weeks to judge. Omalizumab: months
- Watch for
- Cromoglicate: mild tummy upset. Omalizumab: injection-site reactions, rarely allergic reactions.
Specialist territory. Online forums are many things, but not specialists.
Save your money
Popular, heavily marketed or often asked about, but the evidence is weak or says no.
- Strict low-histamine dietsNo proven benefit in MCAS, and a real risk of a shrinking menu and poor nutrition.
- Histamine blood or urine tests and IgG food testsNot reliable for diagnosing MCAS or food allergy, and ASCIA advises against them.
- Kinesiology, hair analysis and Vega testingOn ASCIA’s list of unproven allergy “tests”. Impressive machines, imaginary results.
- DAO enzyme supplementsSmall, low-quality studies in “histamine intolerance”, none in proper MCAS.
- Quercetin and “mast cell stabilising” supplement stacksMostly lab-dish studies. Promising in a test tube is not the same as helping a person.
What you can do
- Keep a diary of episodes and possible triggers.
- Ask your GP for a tryptase request form to keep at home, so blood can be taken 30 minutes to 2 hours after an episode starts.
- Wear medical ID, and carry your adrenaline autoinjector everywhere if prescribed.
- Tell doctors, dentists and anaesthetists before procedures.
See your GP if
- You have repeated episodes of flushing, hives or swelling with faintness, wheeze or tummy cramps
- You’ve been told you have MCAS without a tryptase test during an episode
- Treatment isn’t working, or you’re cutting out more and more foods
- Your symptoms are ongoing rather than in episodes: other causes are more likely and need checking
Call 000 and use your adrenaline autoinjector for any sign of anaphylaxis: trouble breathing, wheeze or persistent cough, swollen tongue or tight throat, hoarse voice, dizziness or collapse. Lie flat (or sit with legs out if breathing is hard), and don’t stand or walk.
Common questions
How is MCAS diagnosed?
All three are needed: typical symptoms in at least 2 body systems at the same time; blood tryptase rising during an episode to more than 20% above your usual level plus 2 (for example, from 5 to 8 or more); and improvement with mast cell medicines. An immunologist or allergist usually makes the diagnosis.
Is MCAS common?
True MCAS is uncommon. Most people with long-term, varied symptoms don’t meet the criteria, so other causes need checking.
Does a low-histamine diet help MCAS?
There’s no proven benefit, and strict diets risk poor nutrition. If you change your diet, see a dietitian with allergy experience.
Are histamine blood or urine tests useful?
No. Histamine blood or urine tests and IgG food tests aren’t reliable for diagnosing MCAS, and ASCIA advises against them.
What are the symptoms of MCAS?
Episodes affecting two or more body systems at once, such as flushing, hives, itch or swelling, wheeze, tummy cramps, diarrhoea, a fast heartbeat or low blood pressure. Symptoms come and go rather than being constant.
Is MCAS the same as histamine intolerance?
No. MCAS is when mast cells release chemicals in repeated episodes, confirmed by tests during an attack. Histamine intolerance is a proposed problem breaking down histamine from food, and the evidence for it is weaker.
What treatments help MCAS?
Non-drowsy antihistamines, sometimes at higher doses, are usually first. Doctors may add famotidine, montelukast or sodium cromoglycate. People who have had severe reactions should carry an adrenaline autoinjector.
Helpful resources
- ASCIA Action Plans for AnaphylaxisThe official plans your doctor fills in, plus travel and first aid plans. Free, Australia and New Zealand.
- Allergy & Anaphylaxis AustraliaNational patient support charity with a helpline (1300 728 000) and practical guides. Australia.
- ASCIA: Evidence-based allergy tests and treatmentsWhich tests are real and which aren’t. Read before paying for one. Free.
Evidence and more information
- ASCIA: Mast cell activation disorders and syndrome, position paper (2025)
- ASCIA: Testing for mast cell activation disorders
- ASCIA: First aid for anaphylaxis
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