How do antihistamines work?

When our bodies feel under attack by a possible threat (like viruses or bacteria) we produce a chemical called histamine. This chemical triggers an immune response that defends us from disease. When an allergy occurs, our body makes histamine in response to something that isn’t a threat at all, like pollen, animal dander, bee stings or peanuts. The release of histamine causes symptoms of an allergic response which can range from mild to life-threatening. 

Allergy symptoms vary depending on the cause (trigger) and how severe the allergy is. Common allergy symptoms include: 

• coughing
• sneezing
• itchy, watery eyes
• blocked or runny nose 
• tingling or swelling of the face, lips, tongue, and throat
• skin rash or hives

Some types of allergies, like food allergies, allergies to certain medications, and insect stings can cause a severe allergic reaction known as anaphylaxis. Anaphylaxis is a medical emergency and can be life-threatening if not treated quickly. Symptoms of anaphylaxis include:

• severe shortness of breath
• difficulty breathing
• palpitations
• feeling dizzy
• nausea (feeling sick)
• vomiting (being sick)
• diarrhoea
• confusion
• skin rash 
• severe swelling of the face, lips, tongue, and throat
• loss of consciousness

Antihistamines work by blocking the effects of histamine in the body and relieving allergy symptoms. They do not cure allergies and only work when you take them. 

How long do antihistamines take to work?

Most antihistamines start working within half an hour of taking them. They are at their most effective after one to two hours. 

How long do antihistamines last?

Different antihistamines last for different amounts of time in your body, and you should read the patient information leaflet included with each carefully to see how often you need to take them. Some antihistamines are short-acting and last for around four to six hours, while longer-lasting varieties can last for twelve or even twenty-four hours.

When should I take antihistamines? 

Ideally, you should take antihistamines before coming into contact with your allergy trigger to prevent symptoms from starting. If you are already suffering from symptoms, taking an antihistamine should make them less severe. If you have a condition such as hay fever where you are likely to be exposed to triggers, it is better to take regular doses of antihistamines as a preventative treatment. 

How many antihistamines can I take in 24 hours? 

How many antihistamines you can take safely in a twenty-four-hour period depends on the type of medication you are taking. Always read the patient information leaflet carefully and do not take more than the recommended dose. Taking too much antihistamine can be harmful to your health. If you think you have taken too much of your medication, seek immediate medical attention. Take the medication packet with you so that doctors can see what and how much you have taken. 

How effective are antihistamines?

Antihistamines are highly effective in treating allergy symptoms when taken correctly. 

Which antihistamine is best?

There are many antihistamines available, and you may need to try a few before deciding which ones work best for you. In general non-drowsy antihistamines are preferable as you will be able to carry on with your daily activities such as working and driving. 

If your allergy is making it difficult for you to sleep, you may be better off taking an antihistamine that makes you drowsy. Some antihistamines work best for certain types of allergies. If you are unsure which antihistamine to take, talk to your GP or pharmacist. 

For insect bites or itchy skin, a non-drowsy oral antihistamine like loratadine (Claritin) can provide relief. If the itching is keeping you awake at night, diphenhydramine (Benadryl) may be better as it can help you fall asleep.

In addition to oral antihistamines, topical antihistamines can be used to relieve itching, and diphenhydramine is available as a cream that you can apply directly to the skin. Applying a cold compress to the bite or sting can also help. Some insect stings like bee stings can trigger anaphylaxis in highly allergic people and need immediate emergency treatment. 

Hives are raised red bumps on the skin that can appear as part of an allergic reaction. The best antihistamine for hives is normally an over-the-counter non-drowsy oral antihistamine like Allegra (fexofenadine) or Claritin (loratadine). Applying a cold compress, taking cool baths, and keeping your skin cool by wearing loose-fitting clothing can also help with hives. 

Pet dander is a mixture of dead cells, saliva, and urine from animal fur, and a common trigger for allergies. Non-drowsy oral antihistamines can work well in controlling symptoms but need to be taken on a regular basis if you are in constant contact with your pet. 

Antihistamine nasal sprays can help with symptoms like sneezing, itchy and runny nose, and eye drops containing antihistamines can provide relief from itchy, watery eyes. As a long-term solution, immunotherapy, which aims to desensitise you to the allergy by exposing you to it over time, may be an option. 

You can minimise your exposure to pet dander by avoiding touching your pet, vacuuming regularly, and running a high-efficiency particulate air (HEPA) filter in your home. 

Hay fever is one of the most common allergies and is triggered by exposure to pollen, commonly tree or grass pollen. It is most common during late spring and summer when the pollen count is highest. There are many over-the-counter hay fever remedies available including oral antihistamines like cetirizine, fexofenadine, and loratadine.

In addition to oral antihistamines, you can also use an antihistamine nasal spray to relieve nasal symptoms and eye drops for itchy, watery eyes. Antihistamines can be taken as required, when you first get symptoms, or as a preventative when the pollen count is high. 

As well as taking antihistamines, you can minimise your exposure to pollen by staying inside with the windows closed, wearing wraparound sunglasses, and dabbing a small amount of petroleum jelly around your nostrils to prevent inhaling the spores.  

If your symptoms are not responding to over-the-counter antihistamines or if your allergies are causing other problems like a worsening of asthma or sinusitis, make an appointment with your GP. 

Antihistamine side effects

Antihistamines can cause side effects in some people. These are usually mild and not serious. Side effects of antihistamines include: 

• nausea (feeling sick)
• vomiting (being sick)
• headache
• dry mouth, nose, or throat
• drowsiness (more common in first-generation antihistamines) 
• reduced coordination and slower reaction times (more common in first-generation antihistamines)
• blurred vision
• difficulty peeing or not being able to pee
• nervousness and irritability
• dizziness
• increased appetite 

Always read the label that comes with your medication before taking it and if you develop any severe side effects stop taking the medication and contact a doctor immediately. 

Who shouldn’t take antihistamines?

Antihistamines are not suitable for everyone. Before you take antihistamines, tell your doctor or pharmacist about any medical conditions you have, and always read the patient information leaflet carefully. Antihistamines may not be suitable for you if you are elderly or  have: 

• diabetes
• epilepsy
• high blood pressure
• an eye condition called glaucoma
• heart disease
• enlarged prostate or difficulty peeing
• kidney or liver problems

Other precautions

Some antihistamines are safe to take if you are pregnant or breastfeeding but tell your GP or pharmacist if you are pregnant or breastfeeding before taking antihistamines. 

Alcohol can increase drowsiness and should be avoided when taking antihistamines. 

Non-drowsy antihistamines can still cause drowsiness in some people. You should not drive or operate heavy machinery if you feel sleepy after taking antihistamines. 

Some medications may interact with antihistamines. Tell your GP or pharmacist about any other medications you are taking, especially: 

• antidepressants
• sleeping tablets
• seizure medications
• narcotics (strong painkillers)
• muscle relaxants 
• sedatives
• indigestion remedies 
• medications for a stomach ulcer

Sources

• NHS: Antihistamines February 28th, 2020, Accessed March 10th, 2022
• Medline Plus: Antihistamines for allergies May 30th, 2020, Accessed March 10th, 2022
• NCBI: Australian Prescriber: Antihistamines and Allergy April 3rd, 2018, Accessed March 10th, 2022
• American College of Allergy, Asthma, and Immunology: Pet Allergies  April 23rd, 2018,  Accessed March 10th, 2022
• NHS Inform: Hay fever November 1st, 2021, Accessed March 10th, 2022
• NHS: Treatment. Insect bites and stings July 8th, 2019, Accessed March 10th, 2022
• https://cks.nice.org.uk/topics/allergic-rhinitis/ (accessed 24/3/22)
• https://www.nhs.uk/conditions/allergies/ (accessed 24/3/22)