Long COVID and Histamine Intolerance, What We Know So Far

Long COVID and histamine intolerance are increasingly discussed together. Some people notice new reactions to foods, drinks, heat or scents after a COVID-19 infection, along with flushing, a racing heart, hives, headaches or digestive upset that were not there before. Researchers have proposed that mast cells, the immune cells that store histamine, may stay overactive after the infection in some people. This is a plausible idea and an active area of research, but it is not yet proven, and long COVID has many other possible explanations. This guide explains what is known, what is still uncertain, how symptoms overlap, and how to work with your doctor on a sensible plan.

The short version is that histamine-type symptoms after COVID-19 are real and worth taking seriously, but they need a proper assessment rather than self-diagnosis. A short, structured low histamine diet can be one tool among several, alongside pacing, sleep and treatment of other long COVID symptoms.

What long COVID is

Long COVID describes symptoms that continue or develop after a COVID-19 infection and are not explained by another cause. The NHS guide to long COVID lists common symptoms including extreme tiredness, breathlessness, joint and muscle pain and problems with memory and concentration. It also lists palpitations, chest tightness, headaches, dizziness, skin rashes such as hives, stomach pain, diarrhoea, nausea, poor sleep, anxiety and low mood, among others.

In the UK, the NICE guideline on the long-term effects of COVID-19 covers how people with new or ongoing symptoms four weeks or more after infection should be assessed, supported and referred for rehabilitation. Many of these symptoms overlap with those of histamine intolerance and mast cell activation, which is one reason the two topics are often linked.

Why mast cells and histamine are being studied

Mast cells sit in the skin, gut lining, airways and around blood vessels. When they are triggered, they release histamine and many other substances that can cause flushing, itching, hives, a blocked nose, a fast heartbeat and digestive symptoms. Several ideas have been proposed to explain why some people develop histamine-type symptoms after COVID-19.

• Ongoing immune activation. In some people, parts of the immune system appear to stay active after the virus has cleared. Mast cells could be part of this.
• Gut lining changes. COVID-19 can affect the gut. Because the enzyme that breaks down food histamine, diamine oxidase (DAO), is made in the lining of the small intestine, gut disruption could in theory reduce how well food histamine is cleared.
• Autonomic nervous system changes. Some people develop problems with heart rate and blood pressure control after COVID-19, which can overlap with histamine-type symptoms.
• Pre-existing tendencies. People who already had allergies, histamine intolerance or a mast cell condition may notice it becomes more obvious after an infection.

These are hypotheses based on small studies, case series and clinical observation. None has been confirmed as the main cause of long COVID, and they may apply only to a subset of people.

Histamine intolerance, MCAS and long COVID compared

The three can overlap. Our comparison of histamine intolerance and MCAS explains the differences in more detail.

Signs that histamine may be part of your picture

There is no single test for histamine-related long COVID. Patterns that may suggest histamine plays a part include:

• New reactions to foods and drinks that are high in histamine, such as wine, aged cheese, cured meats, fermented foods and leftovers.
• Flushing, itching, hives, a blocked nose or a racing heart within an hour or two of eating.
• Symptoms that ease during a short period of fresh, simple meals and return when high histamine foods are reintroduced.
• New sensitivity to heat, alcohol or strong scents alongside food reactions.

None of these rules out another cause. Palpitations, breathlessness and dizziness in particular need medical assessment in their own right.

Other causes doctors check first

Because long COVID symptoms overlap with many conditions, a doctor will usually want to rule out treatable causes before focusing on histamine.

• Heart and lung problems, including inflammation of the heart, blood clots in the lungs and asthma, particularly if you have chest pain, breathlessness or palpitations.
• Anaemia, thyroid problems and vitamin deficiencies, which commonly cause tiredness and palpitations.
• Postural orthostatic tachycardia syndrome (PoTS), which causes a large rise in heart rate on standing. See our article on hEDS, PoTS and MCAS.
• Coeliac disease, inflammatory bowel disease and other gut conditions, if digestive symptoms are prominent.
• Diabetes and other metabolic conditions.
• Anxiety and depression, which are common after any serious illness and can be treated alongside physical symptoms.
• New food allergy, which needs allergy testing if reactions are severe or happen within minutes.

Post-exertional symptom worsening

Many people with long COVID notice that their symptoms get worse a day or two after physical or mental effort, a pattern called post-exertional malaise or post-exertional symptom exacerbation. This can include a flare of histamine-type symptoms, which can make a food seem to be the cause when the real trigger was activity the day before. UK guidance on ME/CFS from NICE no longer recommends graded exercise therapy, and specialists generally advise against pushing through when effort leads to a later crash. Pacing, which means planning activity to stay within your energy limits, is commonly recommended.

For this reason, tracking only food can be misleading. A diary that also records activity, sleep, stress and illness gives a much clearer picture. Our histamine food diary can be used for this, and our guide to histamine intolerance and fatigue covers the overlap with ME/CFS.

A sensible approach to diet

A low histamine diet will not treat long COVID, but if food reactions are part of your symptoms, a short, structured trial can help you find out whether diet makes a difference. It is important to keep it short and planned, because people with long COVID are often already tired, may have a reduced appetite and can easily lose weight on a restrictive diet.

1. Record a baseline for one to two weeks, including food, activity, sleep and symptoms.
2. Simplify for two to four weeks. Focus on fresh foods, cook protein soon after buying, freeze leftovers on the day, and avoid alcohol, aged cheese, cured meats and fermented foods. Our beginner’s guide explains the basics.
3. Keep meals easy. Batch cook on good days and freeze portions. Simple meals such as freshly cooked chicken with rice and vegetables save energy. See our meal prep guide.
4. Reintroduce foods one at a time once symptoms are stable, following our elimination diet plan.
5. Stop and review if the diet is not helping, or if you are losing weight. A dietitian can help.

Eating well when energy is low

Fatigue makes shopping and cooking hard, and a low histamine diet depends on fresh food. A few strategies help.

• Use the freezer. Frozen vegetables, frozen fruit and meat frozen on the day of purchase are convenient and low in histamine.
• Cook once, eat several times. On a better day, cook a larger batch and freeze portions straight away rather than keeping them in the fridge for days.
• Keep simple standbys. Oats, rice, potatoes, eggs, plain rice cakes, fresh milk and frozen peas make quick meals with little effort.
• Accept help. Ask family or friends to cook fresh meals or help with shopping, and use online deliveries if they suit you.
• Sit down to prepare food. A stool in the kitchen saves energy, which matters if you have post-exertional symptoms or PoTS.
• Watch your weight and appetite. If you are losing weight without trying, tell your doctor and ask about a dietitian.

Our one-pan dinners and slow cooker recipes are designed to keep effort low.

New infections and vaccination

Some people with long COVID notice that a new infection, whether COVID-19 or another virus, brings a temporary flare of symptoms, including histamine-type reactions. Simple steps to reduce infections, such as hand washing and avoiding close contact with people who are unwell, are sensible. Vaccination is recommended for many people with long COVID, according to national programmes. If you have ever had a severe allergic reaction, or you have a diagnosed mast cell condition, discuss vaccination with your doctor beforehand. Vaccines are given in settings prepared to manage allergic reactions, and a history of histamine intolerance alone is not usually a reason to avoid vaccination.

Children and young people

Children and teenagers can also have long COVID, and their symptoms may look different from those in adults, with tiredness, headaches, stomach aches and problems at school being common. Do not put a child on a restricted diet without advice from their doctor, because children need a varied diet to grow. Our guide to histamine intolerance in children explains what to discuss with their doctor.

Medicines and supplements

Some doctors treating people with long COVID and histamine-type symptoms use antihistamines, sometimes combining H1 and H2 types, and specialists may consider prescription mast cell stabilisers such as sodium cromoglicate. The evidence for these in long COVID comes mainly from small studies and clinical experience. They should be used only with medical advice, because the right choice depends on your symptoms, your other medicines and your health. We do not give doses.

Supplements such as quercetin are widely promoted online for mast cells, but human evidence is limited, and they can interact with medicines. Our guides to quercetin and mast cell stabilizers explain what is known. Vitamin C is often described as a DAO cofactor, but it is not one, and the role of vitamin B6 is unproven. DAO depends on copper and a cofactor built into the enzyme itself. High-dose B6 can damage nerves.

Never stop or change a prescribed medicine to make room for a supplement or because it appears on a list of possible DAO inhibitors. Our guide to medications that may block DAO explains the evidence behind those lists.

Does it get better?

Many people with long COVID improve gradually over months, although the pace varies a great deal and some people have symptoms for much longer. There is no reliable way to predict how long histamine-type symptoms will last. Some people notice flares with new infections, periods of stress or overexertion. Keeping a record over time helps you see whether things are slowly improving, even when individual weeks feel unchanged. Our article on whether histamine intolerance can be cured discusses how acquired histamine problems can change over time.

Looking after your mental health

Living with symptoms that are hard to explain, fluctuate from day to day and are not always taken seriously is exhausting. Many people with long COVID feel anxious about food, worry about every new symptom or feel isolated when they can no longer eat out or socialise as before. These feelings are understandable, and they are not a sign that symptoms are imagined. Talking therapies, peer support groups and long COVID services can help, and treating anxiety or low mood often makes physical symptoms easier to manage too. If food has started to feel frightening, or your list of safe foods keeps shrinking, mention this to your doctor, as a dietitian or psychologist can help you widen your diet safely.

Tests and what they can show

Routine blood tests are often normal in long COVID, which can be frustrating but is still useful, because it rules out several treatable conditions. Serum DAO tests are sold widely but are not validated for diagnosing histamine intolerance. Tryptase is a blood test used in the assessment of mast cell disorders, and it is most useful when taken during or soon after a severe reaction. Our guides to histamine intolerance tests and misleading histamine tests explain the options.

Preparing for your appointment

A clear timeline is one of the most helpful things you can bring. Note when you had COVID-19, when new symptoms appeared, which symptoms bother you most, any foods or triggers that seem consistent, and whether activity makes things worse a day or two later. Bring a list of all your medicines and supplements. Ask whether a referral to a long COVID service, a cardiologist, an allergist or a dietitian would help.

When to see a doctor

Call emergency services straight away if you or someone else has swelling of the lips, tongue or throat, difficulty breathing or wheezing, a hoarse voice, feeling faint or collapse. These are signs of anaphylaxis, which needs an adrenaline (epinephrine) injection and urgent care.

Also call emergency services for chest pain, severe breathlessness, coughing up blood, fainting, confusion, or signs of a stroke such as face drooping, arm weakness or slurred speech. Seek urgent same-day care for pain or swelling in one leg, which can be a blood clot. See your doctor if symptoms continue four weeks or more after COVID-19, if you have new palpitations, ongoing breathlessness, unexplained weight loss, or if low mood or anxiety are affecting your life. If you are having thoughts of harming yourself, contact a crisis line or emergency services. Never stop a prescribed medicine without medical advice.

Long COVID and histamine questions answered

Can COVID-19 cause histamine intolerance?

Some people develop histamine-type symptoms after COVID-19, and researchers have proposed that ongoing mast cell activation or gut changes may play a part. This has not been proven, and other causes of long COVID symptoms need to be ruled out first.

Is long COVID the same as MCAS?

No. Mast cell activation is one proposed mechanism behind some long COVID symptoms, but long COVID is a broader condition with several possible causes. MCAS has its own diagnostic criteria and needs specialist assessment.

Will a low histamine diet help long COVID?

It may help if food reactions are part of your symptoms, but it does not treat long COVID itself. Keep any diet trial short and structured, and stop if you are losing weight or not seeing a benefit.

Are antihistamines used for long COVID?

Some doctors use antihistamines for histamine-type symptoms after COVID-19, based on small studies and clinical experience. Ask your doctor or pharmacist whether they suit you, and do not combine medicines without advice.

Why do my food reactions get worse after a busy day?

Many people with long COVID have post-exertional symptom worsening, where effort leads to a flare a day or two later. This can make foods seem like the cause. Tracking activity alongside food helps you tell the difference.

Which tests should I ask about?

Your doctor will usually start with routine blood tests and checks of your heart, lungs and blood pressure. Tryptase and allergy tests may be considered depending on your symptoms. Serum DAO tests are not reliable for diagnosis.

Medical disclaimer: This content is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider before starting any supplement regimen.