The first three steps

Safe initial steps to try to relieve MCAS symptoms

Disclaimer: The steps detailed below have NOT been written by a medical doctor. They have been written by an Irish MCAS patient who resides in the Republic of Ireland and has no medical background nor medical qualifications. The intent is to offer basic well intentioned information in good faith that may help patients who suspect they may have MCAS but who do not yet have access to a suitable MCAS clinician.

Many published reviews of mast cell disease which talk about therapeutic management of the disease state that early introduction of H1 & H2 blocking is a standard step in the management of mast cell diseases of any type.

Take particular care to read the medication information sheets included with your medications. Pay careful attention to recommended dosage and be on guard for potential side effects.

If you live in Ireland and suffer an adverse reaction to ANY medication, please take the time to make a “Human Medicine Adverse Reaction” report to the Health Products Regulatory Authority by following this link.

If you live outside of Ireland, it is likely there will be a similar authority in your jurisdiction that collates adverse medicine reactions.

Step 1 – Identify and avoid triggers (Most Important)

Work to “Identify & Avoid” your activation triggers in what you eat, drink, breathe & wear. Some triggers are unavoidable and some unfortunate MCAD patients react to noise, vibration, temperature changes, temperature extremes and even variations in atmospheric pressure.

Many activation triggers, however, are chemical substances that we experience in our daily lives at home, at work or even while travelling. The link below details some common triggers in a printable A4 sheet but bear in mind triggers will vary from person to person and some may be unique to you.

Subsequent steps cannot work to their best effect if you do not avoid triggers as best as possible. You cannot put out a fire if you keep pouring petrol upon it.

Step 2 – Trial all available H1 antihistamines

All H1 antihistamines are NOT the same. There are multiple different H1 antihistamines and indeed multiple different generations. Some are H1 receptor antagonists while others are reverse agonists and all work through slightly different mechanisms. Take note that just because one particular H1 antihistamine may be ineffective, it does not mean that another will not offer some benefit.

Obtain a list from your pharmacist of all OTC & prescription H1 antihistamines and then trial them one at a time for 2-4 weeks to see if they help alleviate some of your symptoms. Some symptoms that H1 antihistamines have shown to alleviate include anxiety, pain & skin rashes. Be aware that for some patients, a given antihistamine may exacerbate some symptoms.

For trialling prescribed medications you will need to work with your family doctor.  If it is very apparent that a particular antihistamine is causing serious side effects or is useless, ditch it immediately and move on to the next one on your list.

Once you find the most suitable H1 antihistamine, and this may take many months, stay on it at the recommended dose. The most suitable antihistamine may end up being the very first one you tried, but you will not know this until you have tried all of the others.  Don’t work off hearsay; something with no benefit for someone else may be very beneficial for you.

Some examples of H1 antihistamines include azelastine, bilastine, cetirizine, chlorphenamine, desloratadine, fexofenadine, levocetirizine, loratadine, promethazine but there are many more and availability varies by jurisdiction.

Some H1 antihistamines may have a sedating effect which may affect your ability to drive or operate machinery. With this in mind it is obvious that non-sedating antihistamines are preferred.

However, if it turns out that a sedating type H1 is all that offers you benefit, a workaround can include taking such medication before bedtime.

Your pharmacist or GP will be best placed to assist you to determine which antihistamines are likely to cause a sedating effect and always read thoroughly the information leaflet that accompanies any medications you take, be they OTC or prescribed.

We have purposely avoided the use of brand names for describing H1 antihistamines because we are aware of some cases where a single brand name actually refers to completely different active ingredients in different jurisdictions. For example Benadryl in the USA is the H1 antihistamine diphenhydramine. In Ireland, Benadryl is a retail range of allergy medicines contain widely differing ingredients, some of which contain no antihistamines at all.

Step 3 – Trial all available H2 antihistamines

Whilst continuing to take the most effective H1 antihistamine for you, trial all the available H2 antihistamines.

This is a very short list: cimetidine (Tagamet), famotidine (aka Pepcid), ranitidine (aka Zantac) and nizatidine (aka Axid, Tazac). As you may already be aware Pepcid and Zantac are available OTC but are also available on prescription. Both cimetidine & nizatidine are “prescription only” in Ireland.

Be careful with cimetidine and work carefully with your pharmacist to check for interactions with other medication you may be taking as it has a long list of known interactions due to the metabolic pathways it uses.  

World events have caused serious supply problems for H2 antihistamines. Firstly, ranitidine (Zantac) has been recalled almost worldwide due to a contamination scare. This has caused ranitidine users to switch to other H2s and has led to severe supply pressure for the remaining H2s.

Secondly, famotidine has apparently shown some benefit treating COVID-19 patients and this has caused a run on this particular medicine. It is very likely that at the date of writing (May 2020) your local pharmacist will have no OTC H2 antihistamines and will likely not be in a position to order any fresh stock of even prescription ones.

Some pharmacies may have old stock of cimetidine so let your fingers do the walking and phone around.

Reasons for taking both a H1 & a H2 antihistamine in combination

A receptor is a region of tissue or a molecule in a cell membrane which responds specifically to a particular neurotransmitter, hormone, antigen or other substance.

Mast Cells and other cells that respond to histamine can feature both H1 & H2 histamine receptors; but some cells feature only H1 receptors and other cells feature only H2 receptors.

A key point to understand is that EITHER of these receptors can be activated by histamine docking on either one. So, if you are taking a H1 antihistamine, this blocks the H1 receptor and prevents it from activating.

But if you are not taking a H2 antihistamine, the H2 receptor is still free to be activated potentially causing degranulation of your mast cells. That is why you need to trial blocking both receptors.

Think of a mast cell as a hand grenade with 2 fly off handles, you need to keep both pins in to keep the grenade safe.

Do NOT confuse H2 antihistamines with Proton Pump Inhibitors

H2 antihistamines have traditionally been used to treat heartburn and excess stomach acid. A newer class of drugs have come to market in recent decades that are more effective at this task and these are called Proton-pump Inhibitors (PPIs). PPIs include include drugs like Losec & Nexium etc.

In the 3 steps above we are seeking to trial H2 antihistamines solely for their H2 receptor blocking properties. We are NOT trying to treat stomach acid problems.

Furthermore, PPIs are NOT antihistamines and are totally useless for what we are trying to achieve which is a reduction in symptoms caused by Mast Cell degranulation.

A note of caution about sulphite intolerance

Most hard tablets are bulked using maize starch. The first step when processing corn in order to make maize starch involves soaking the corn nibs in sulphuric acid. This step softens or removes the thick skin and make subsequent processing steps easier. This initial step introduces sulphites into the maize starch which offers a biocidal preservative effect.

Anecdotally, it appears many MCAS patients suffer an intolerance to sulphite preservatives. In the EU and in the USA, sulphites & sulphur dioxide (SO2) are classed together as a single allergen under food labelling laws.

Somewhat unique in terms of a known allergen, sulphites & SO2 are legally allowed to be hidden in food, drink & medication as long as their concentration is less than 10 milliliter per litre or less than 10 milligrams per kilogram.

We suspect that legally allowing known allergens to be hidden in food, drink or medication may prove to be one of humanities bigger “own goals”.

Some antihistamines such as bilastine or cimetidine are physically large tablets and this consequently means they have a lot of maize starch which may contain larger amounts of hidden sulphites. Other antihistamines like chlorphenamine (Piriton) are physically small but you may need to take many of them to achieve a benefit.

If you find two antihistamines that are equally effective, it is recommend you chose the one with the least mass in case triggering by trace sulphites is an issue for you. Capsule tablets may be less likely to have hidden sulphites.


The above steps are hard work and require good organisation & discipline. Keep written notes on effectiveness if it helps. Only make one change at a time.

The above steps will take many months to effect but if you find the right combination the benefit will hopefully be an improved quality of life. If you have any doubt at any stage about the safety of the above steps please consult with your doctor or pharmacist.

What we are trying to achieve is more good days than bad days.

The Next Steps with MCAS.

The next steps may include mast cell stabilisers like ketotifen or sodium cromoglycate, but for these steps you will most definitely need to be under the care or guidance of a suitably qualified medical doctor who understands MCAS.

Gavin Tobin – Kildare, Ireland – 1st May 2020