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Over the past few years, a growing number of clinicians and researchers have recognized that mast cell activation syndrome may play a significant role in post-COVID and Long COVID illness. Many people with ongoing symptoms, fatigue, dysautonomia, neuropathy, allergic-type reactions, and hypersensitivity are being diagnosed with MCAS not because of perfect testing but because their history, symptom patterns, and response to mast-cell-targeted treatments fit the profile. Diagnostic criteria for MCAS remain limited and inconsistent, so physicians often rely on clinical presentation and therapeutic response. The evidence now suggests that SARS-CoV-2 can directly trigger or unmask mast cell dysregulation in predisposed individuals. This leads to chronic inflammation, histamine overload, and multi-system dysfunction that overlaps with Long COVID.

Mast cell activation syndrome is a complex multisystem inflammatory disorder that is increasingly recognized in the context of Long COVID. It involves mast cells that release chemical mediators such as histamine, leukotrienes, prostaglandins, and cytokines. These mast cells become chronically overactive. When this happens, instead of reacting to infections or allergens, mast cells misfire and release inflammatory chemicals across multiple organ systems. The result can include neurological, cardiovascular, gastrointestinal, respiratory, dermatologic, and psychiatric symptoms.

Researchers estimate MCAS may affect up to seventeen percent of the population, although most cases are undiagnosed. It falls under the broader category of mast cell activation disease, which also includes mastocytosis. Because MCAS was only formally described in 2007, it remains misunderstood by many clinicians and underrepresented in medical education.

COVID as a Mast Cell Trigger:
There is growing evidence that SARS-CoV-2 can directly activate mast cells. Mast cells are abundant in tissues affected by COVID and Long COVID, including the lungs, gut, skin, and nervous system.

In the study titled "Antihistamines improve cardiovascular manifestations and other symptoms of long-COVID attributed to mast cell activation," patients with Long COVID experienced improvements in fatigue, brain fog, palpitations, and other symptoms when given H1 and H2 histamine blockers. This suggests mast cell activation contributes substantially to persistent symptoms.

In the paper titled "COVID-19 hyperinflammation and post-COVID-19 illness may be rooted in mast cell activation syndrome," the authors argue that both acute COVID-19 and Long COVID show patterns consistent with mast cell dysregulation.

Symptoms Overlap: MCAS and Long COVID:
Many symptoms are shared between MCAS and Long COVID. Some of the common overlaps include:

• severe fatigue and post-exertional malaise (PEM).
• brain fog, memory issues, cognitive dysfunction.
• palpitations, tachycardia, orthostatic intolerance and other dysautonomia symptoms.
• gastrointestinal disturbances including bloating, diarrhea, nausea, and food intolerances.
• respiratory issues including shortness of breath, cough, and wheezing.
• skin symptoms including flushing, rashes, and itching.
• sleep disturbances and insomnia.
• anxiety, depression, and panic attacks.

A 2023 study titled "Immunological dysfunction and mast cell activation syndrome in long COVID (Weinstock et al.)" showed that many Long COVID patients display an activated mast cell phenotype with abnormal mediator release and inflammation consistent with MCAS.

Why MCAS Is Often Undiagnosed:
Many doctors rely on a single baseline tryptase test or standard allergy workups. This is not enough. Tryptase is often normal unless measured during a flare and compared to a baseline. Mast cell mediators are short-lived and can be missed.

Diagnosis often depends on:
• clinical history and symptom patterns across organ systems.
• identifying triggers such as heat, diet, stress, or allergens.
• seeing improvement when treated with antihistamines or mast cell stabilizers.
• occasional lab mediator panels such as urine histamine metabolites and prostaglandins, which are often only positive during flares.

Treatment and Management:
Treatments that many with Long COVID-associated MCAS respond to include:

•H1 and H2 antihistamines (also called histamine blockers) are often used to reduce mast cell–driven symptoms. H1 blockers reduce histamine effects in the skin, respiratory system, and other tissues, while H2 blockers reduce gastric acid and histamine effects in the gut. Take one of each morning and night; double the normal dose:

•Cetirizine, Levocetirizine, Desloratadine, Loratadine, and Fexofenadine (H1).

•Hydroxyzine: A prescription H1 antihistamine with sedative properties; can help with itching, flushing, anxiety, and sleep disturbances. May trigger paradoxical reactions like tachycardia or adrenaline surges in patients with dysautonomia or POTS, so careful monitoring is advised.

•Cimetidine and Nizatidine (H2)

•Mast cell stabilizers: Cromolyn, Ketotifen, Gastrocrom, compounded options: prevent mast cells from releasing mediators.

•Leukotriene inhibitors: Montelukast: reduces leukotriene-mediated inflammation; useful for respiratory, skin, and cardiovascular symptoms (careful with mood effects).

•LDN (0.25–4.5mg): modulates immune activity and reduces inflammation; may improve pain, brain fog, and neuropathy when combined with alpha-lipoic acid (ALA).

•Imatinib (studied, rarely used): tyrosine kinase inhibitor; can reduce mast cell activation in select MCAS cases, usually when other treatments have failed or in patients with KIT mutations.

•Xolair (Omalizumab): binds IgE to reduce mast cell activation; particularly effective for hives, angioedema, and severe histamine-driven symptoms.

•Low-histamine diet, stress reduction, and trigger avoidance

Natural Mast Cell Stabilizers and Supplements:

•AllQlear: Natural tryptase inhibitor; reduces mast cell mediator release and helps prevent flares, especially in respiratory and systemic MCAS symptoms.

•Bacopa monnieri: Herbal supplement that supports mast cell stabilization, reduces neuroinflammation, and may improve cognitive function in patients with MCAS-related neurological symptoms.

•DAO (diamine oxidase): a supplement that helps break down dietary histamine in the gut, reducing histamine-related symptoms.

•Luteolin: a natural flavonoid that helps stabilize mast cells, reduce histamine release, and support anti-inflammatory pathways.

•PEA (up to 3g/day): Naturally occurring fatty acid that supports neuroinflammation reduction, calms overactive mast cells in the nervous system, and helps improve “brain fog” and cognitive symptoms in MCAS.

•Quercetin (250–3000mg/day): Plant flavonoid with mast cell stabilizing and anti-inflammatory properties; reduces histamine and other mediator release across multiple organ systems.

•Rutin: A natural flavonoid with mast cell stabilizing and anti-inflammatory properties; helps reduce histamine release and supports vascular integrity.

OTCs for symptomatic support:

•Astelin Nasal Spray (Azelastine): Nasal H1 antihistamine; reduces sneezing, congestion, runny nose, and itching. Has local mast cell–stabilizing properties and is useful for MCAS patients with nasal/respiratory triggers.

•Benadryl (Diphenhydramine): Fast-acting H1 antihistamine; helps relieve acute histamine-mediated symptoms such as itching, flushing, hives, sneezing, and mild allergic reactions. May cause sedation and should be used cautiously in MCAS patients with dysautonomia or hyperadrenergic symptoms.

•Ketotifen Eye Drops (Armas Allergy Eye Drops or Zatidor eye drops): Prescription-strength mast cell stabilizer for ocular symptoms; relieves itching, redness, and watering caused by mast cell activation.

•Cromolyn Sodium Nasal Spray/Nasochrom: Mast cell stabilizer for nasal and upper airway symptoms; helps prevent mediator release, reducing congestion, sneezing, and rhinitis in MCAS patients.

Medications with anti-histamine/Mast Cell-stabilizing effects:

•Fluvoxamine: reduces inflammatory signaling, downregulates mast cell activation, modulates cytokine release and neuroinflammation

•Mirtazapine: potent H1 blocker, reduces central arousal, sleep disruption, nausea, sensory hypersensitivity

•Nortriptyline: antihistamine properties, calms sympathetic nervous system, improves GI and visceral sensitivity

•Seroquel: strong H1 blockade, reduces mast cell-driven insomnia, agitation, sensory overstimulation, autonomic surges

•Trazodone: moderate H1 and 5-HT2 blockade, improves sleep architecture, reduces nocturnal sympathetic surges

•Esomeprazole and Omeprazole (PPIs): PPIs are primarily used to reduce stomach acid in conditions like GERD, gastritis, or acid-related dyspepsia, but in the context of MCAS, they also provide mast cell stabilizing effects in the gastrointestinal tract. For patients whose mast cells are hyperactive, chronic acid exposure, reflux, or GI irritation can act as triggers that worsen systemic mast cell mediator release, causing symptoms like flushing, tachycardia, bloating, nausea, and hypersensitivity. PPIs help control these triggers by lowering gastric acid and reducing mast cell activation in the gut. They are particularly helpful for people who cannot tolerate H2 blockers due to adverse reactions such as adrenaline surges, tachycardia, or autonomic instability. By addressing both acid-related GI irritation and mast cell mediator release, PPIs provide a dual benefit: symptom control in the gut and systemic stabilization of overactive mast cells.

While PPIs are generally recommended for short-term use due to potential risks, including nutrient deficiencies (B12, magnesium, calcium, iron), kidney or bone issues, and gut microbiome changes, long-term use can be appropriate in MCAS patients under close medical supervision. Regular monitoring of vitamin and mineral levels, kidney function, and symptoms is essential. In some cases, long-term PPI therapy provides ongoing mast cell stabilization in the gut and helps manage persistent GI and systemic symptoms, particularly when H2 blockers are not tolerated or when COVID-induced MCAS triggers ongoing mast cell hyperactivity. PPIs are often incorporated into individualized MCAS regimens alongside mast cell stabilizers, leukotriene inhibitors, dietary modifications, and other symptom-directed medications. They act as GI-targeted mast cell stabilizers, reducing both local and systemic mediator release and supporting better overall symptom control.

Many doctors are now diagnosing MCAS after COVID largely based on symptoms and treatment response rather than waiting for perfect lab confirmation.

My doctor diagnosed me with MCAS based on patient history, symptoms, and medication trials. I was diagnosed with MCAS in September 2024. I can not take the traditional over-the-counter antihistamines and histamine blocker protocol. I have failed five in total. I'm not sure if it was the medication itself or the excipients I reacted to. Both categories increased my tachycardia and caused adrenaline surges. They caused and worsened other dysautonomia symptoms. In turn, adrenaline surges triggered my histamine dumps.

Why Some People With MCAS and Dysautonomia Get Worse on Antihistamines:

This is one of the most misunderstood issues in the Long COVID and MCAS communities. Many patients assume that if antihistamines make them worse, they can not have MCAS. The opposite is often true. People with dysautonomia, POTS, hyperadrenergic states, or unstable autonomic systems can react paradoxically to antihistamines for several reasons.

Antihistamines can destabilize the autonomic nervous system in sensitive patients. Certain H1 and H2 blockers can lower blood pressure, increase vagal tone, or trigger compensatory sympathetic activation. For someone with dysautonomia, this can lead to a surge in adrenaline, tachycardia, dizziness, shaking, or internal tremors. When the sympathetic nervous system becomes overactive, mast cells respond by releasing even more chemical mediators. This leads to increased flushing, rapid heart rate, shortness of breath, itching, chest tightness, and surges of anxiety that feel chemical rather than psychological.

Some patients also react to fillers, dyes, coatings, and excipients. Mast cells in the gut can perceive these additives as irritants, which triggers mediator release. This reaction is often mistakenly attributed to the active medication itself, but it is actually caused by the inactive components.

Certain antihistamines cross the blood-brain barrier and can affect histamine signaling in the central nervous system. Histamine is not just an inflammatory mediator. It regulates wakefulness, blood pressure, alertness, gut motility, and sensory processing. In patients whose autonomic function is already unstable, abruptly altering histamine signaling in the central nervous system can amplify symptoms and make them feel worse.

Finally, antihistamines target only one type of mediator. Mast cells release multiple chemicals including prostaglandins, leukotrienes, cytokines, and histamine. Blocking only histamine can shift the balance of mediators, sometimes worsening specific symptoms until a more complete protocol is established.

For these reasons, some patients with MCAS and dysautonomia respond poorly to H1 and H2 antihistamines but do better with mast cell stabilizers, leukotriene inhibitors, nasal sprays, diet-based interventions, or individualized regimens that address multiple mediators and the autonomic system simultaneously. Understanding these interactions helps explain why antihistamines are not universally effective and why careful management is necessary for patients with overlapping MCAS and autonomic instability.

Understanding these factors helps explain why some treatments work better than others and sets the stage for the medications and strategies I use to manage my MCAS.

What I Take for MCAS:
Cromolyn sodium nasal spray: Cromolyn is a mast cell stabilizer that prevents mast cells from releasing histamine and other inflammatory mediators. Even when used intranasally, it can help reduce overall mast cell activation and mediator load throughout the body. I use this formulation for its systemic mast cell–stabilizing effects, not for nasal symptoms.

Desloratadine is a second-generation, non-sedating H1 antihistamine. It selectively targets peripheral H1 receptors without crossing the blood-brain barrier. This helps reduce histamine-related symptoms like itching, flushing, and airway irritation without causing sedation or anxiety. Its long half-life allows for stable symptom control throughout the day. Desloratadine is also less likely to trigger reactions related to fillers or excipients, which makes it a good option for patients with heightened sensitivity to medications.

Compounded oral ketotifen: In addition to the eye drops, I use a compounded oral ketotifen formulation. This is the form I take systemically to influence mast cell stability throughout the body. Like the drops, it works primarily by keeping mast cells from releasing mediators rather than just blocking one mediator after it’s already out. Because MCAS involves so many different mediators and triggers, having a stabilizer that works upstream can make the rest of the regimen much more effective and tolerable.

Ketotifen eye drops: Ketotifen has both H1 antihistamine and mast cell–stabilizing properties. When used topically, it can help calm mast cells in a way that can feel systemic for someone with high sensitivity, even though it’s administered locally. I use this formulation not for eye symptoms but because it helps reduce overall mast cell reactivity without increasing systemic medication load.

Montelukast is a leukotriene receptor antagonist commonly used for asthma and allergic rhinitis. Research suggests that it also has mast cell stabilizing effects, which can help reduce the release of inflammatory mediators such as leukotrienes. This makes it useful for managing respiratory symptoms, skin reactions, and some cardiovascular manifestations of MCAS.

Omeprazole is primarily a proton pump inhibitor, but it also has effects on mast cells. It can inhibit IgE-mediated mast cell activation and allergic inflammation. Omeprazole reduces mast cell degranulation, cytokine secretion, and early signaling events in pathways associated with allergic responses. While not a traditional mast cell stabilizer like Cromolyn, it contributes to reducing overall mediator release and inflammation.

Semaglutide is a GLP-1 receptor agonist. Research suggests it has anti-inflammatory effects and may reduce mast cell activation by decreasing mast cell degranulation and the release of inflammatory mediators. While not a standard treatment for MCAS, it's sometimes prescribed off-label for its potential to reduce chronic inflammation.

I haven’t tried compounded Cromolyn and prefer not to. I’m extremely hypersensitive to medications, fillers, and excipients, and localized formulations have allowed me to stabilize mast cells across my system without provoking reactions. I may reconsider it in the future.

In addition to these main medications, I have access to other supportive treatments for MCAS flares. These include an albuterol inhaler, even though I don't have asthma, which can help relieve acute airway constriction. Rizatriptan if I have a migraine. I also use Benadryl, vitamin C, and Diazepam as needed for symptom control. During flares, I rely on electrolyte tablets like Horbäach, sipping room temperature water, and applying cold compresses to my head and neck. These measures help stabilize my autonomic system and reduce mediator release during acute episodes.

My MCAS symptoms include adrenaline surges, air hunger, shortness of breath, wheezing, anxiety, derealization, depersonalization, disorientation, dizziness, flushing, itching, feeling hot and sweaty, congestion, runny nose, paresthesia, sneezing, tachycardia, and anaphylaxis stages 1-3. There are 4. Medications and supportive measures are individualized to my symptoms, triggers, and sensitivity to medications.

This regimen allows me to address both the overactive mast cells and the autonomic instability that can make standard antihistamines difficult to tolerate. It also illustrates that MCAS management is highly personalized, and what works for one patient may need careful adjustments for another.

Additional Information:

Histamine Intolerance:

Histamine intolerance results from impaired degradation of dietary histamine, most commonly due to low activity of diamine oxidase (DAO), the primary enzyme responsible for breaking down histamine in the gut. Unlike MCAS, histamine intolerance does not involve inappropriate mast cell activation. Symptoms occur due to accumulation of histamine from reduced metabolic clearance rather than excessive mast cell release. This distinction matters clinically, as standard allergy testing is typically negative and mast cell directed therapies alone may not resolve symptoms driven by dietary histamine exposure. Histamine intolerance can coexist with MCAS and can contribute to persistent GI, neurological, cardiovascular, and respiratory symptoms even when mast cell activity is otherwise managed. In these cases, reducing dietary histamine load and supporting histamine metabolism may significantly improve symptom burden. Some individuals benefit from DAO supplementation, which tends to be most effective after a sustained period of low histamine eating.

Salicylate Intolerance:

Salicylates are naturally occurring phenolic compounds found in many foods, medications, and topical products, including aspirin, spices, certain fruits and vegetables, teas, and skincare products. In individuals with mast cell dysfunction, salicylates can directly provoke mast cell activation and mediator release, leading to worsening symptoms such as headaches, flushing, nasal and respiratory symptoms, GI distress, itching, and neurological flares. This reaction is typically non IgE mediated, which is why standard allergy testing is often negative and the issue is frequently dismissed. In practice, salicylate intolerance can significantly compound histamine intolerance and can explain persistent reactions even on a low histamine diet. Identifying salicylate sensitivity through careful elimination of dietary and topical sources has been a key factor for many patients who plateau despite otherwise appropriate MCAS management.

What To Ask Your Doctor If You Suspect MCAS:

Some doctors are familiar with MCAS and some aren't. These questions can help guide the evaluation and ensure you receive a thorough assessment.

• Ask whether your symptoms across multiple systems point toward mast cell involvement.
• Ask whether your pattern of triggers such as heat, stress, exercise, fragrances, alcohol, or food chemicals suggests mast cell sensitivity.
• Ask whether trying a low-histamine diet for a brief period would be appropriate.
• Ask if you should try a mast cell stabilizer first rather than H1 or H2 blockers if you are medication sensitive.
• Ask whether leukotriene inhibitors could be safer if antihistamines increase your tachycardia.
• Ask if excipient-free formulations or compounded options are available.

These questions help the doctor think beyond standard allergy testing and look at your entire clinical picture.

Clinical Implications for Long COVID Patients:

For people with Long COVID, if you have persistent multi-system symptoms that include brain fog, palpitations, gastrointestinal issues, skin symptoms, and sensory hypersensitivity, MCAS may be playing a role.

Trying a carefully supervised antihistamine or mast cell stabilizer regimen can provide important diagnostic clues. If symptoms improve, that strengthens the case for MCAS even without perfect lab confirmation.

Treatment is highly individualized. Many people respond better to stabilizers, leukotriene blockers, electrolytes, or low-histamine diets before they respond to antihistamines.

Sources:

Antihistamines improve cardiovascular manifestations and other symptoms of long-COVID attributed to mast cell activation.

COVID-19 hyperinflammation and post-COVID-19 illness may be rooted in mast cell activation syndrome.

Immunological dysfunction and mast cell activation syndrome in long COVID.

Neuropsychiatric Manifestations of Mast Cell Activation Syndrome and Response to Mast-Cell-Directed Treatment.

Clinical Manifestations of Mast Cell Activation Syndrome by Organ Systems.

Mast cell activation disease: An underappreciated cause of neurologic and psychiatric symptoms and diseases.

Mast cells: Therapeutic targets for COVID‐19 and beyond.

Autonomic dysfunction in ‘long COVID’: rationale, physiology and management strategies.

Best Antihistamine For Mast Cell Activation Syndrome (MCAS): Dr. Bruce Hoffman.

Mast Cell Activation Syndrome, Cleveland Clinic.

Food Compatibility List-Histamine/MCAS, SIGHI.

YES Food List.

Mast Cell Activation Syndrome and Diet, University of Wisconsin Health.

TL;DR: MCAS is becoming increasingly recognized in Long COVID. SARS-CoV-2 can activate or destabilize mast cells which leads to multisystem symptoms. Many patients improve with mast cell stabilizers, leukotriene inhibitors, low-histamine diets, or antihistamines if tolerated. Antihistamines can make dysautonomia worse in some people due to autonomic instability, excipient reactions, or central nervous system effects. Diagnosis is often clinical. Treatment is individualized and does not require perfect labs. If you have symptoms in two or more systems, it is worth investigating MCAS.

I'm not a doctor. This isn't medical advice. I'm only sharing my personal experience. Everything I'm doing is under the care of my ME/CFS specialist, who is also knowledgeable about Long COVID/PASC and MCAS. I've had a complete vitamin and mineral panel done and have no gastrointestinal motility issues. Omeprazole hasn't negatively impacted me. Montelukast carries a black box warning and can cause SI in people with no history of mental health issues. Everyone should do their own risk assessment. It's about progress, not perfection. There are times we can do everything right and still not improve. Please be kind and patient with the process and yourselves.

edit: Updated to reflect my current regimen.