Antihistamines: Uses and side effects

What is an antihistamine & how does it work?

An antihistamine is a type of medication that stops or reduces the symptoms of hay fever such as a blocked nose, a headache, and itchy eyes. Antihistamines do not cure hay fever, but they can be used to manage it. If you’re prone to hay fever or allergies, antihistamines can be taken to prevent symptoms when you are exposed to pollen.

Antihistamines work by blocking histamines, the chemical your body produces in response to an allergen. Histamines causes the skin to swell and expansion of the blood vessels, which is at the root of most hay fever symptoms. Antihistamines stop this from happening, which means that you should notice a decrease in swelling, itching, and other symptoms.

Types of antihistamines

The primary types of antihistamines are drowsy and non-drowsy. There are different medications in each class of antihistamines, and each one works slightly differently. If you find one does not work for you, you could try a different one.

Drowsy antihistamines

Drowsy antihistamines can cause fatigue and drowsiness. These aren’t recommended if you need to drive or operate heavy machinery after taking this medication, but they could help you sleep if hay fever symptoms interfere with your sleep patterns. Drowsy antihistamines include medications such as chlorphenamine and promethazine.

Non-drowsy antihistamines

Non-drowsy antihistamines are less likely to make you sleepy so shouldn’t interfere with your day-to-day routine. Non-drowsy antihistamines include medications such as loratadine and cetirizine.

Forms of antihistamine medication

There are different ways to take antihistamines depending on the symptoms you want to alleviate:

●      Antihistamine tablets target the whole body and can help to reduce common symptoms of hay fever.         

●      Antihistamine eyes drops can help reduce eye swelling and itchiness.

●      Antihistamine cream can help to reduce allergic swelling and rashes on the skin. An antihistamine cream can be used for general swelling due, or more acute rashes caused by direct contact with an allergen.

●      Antihistamine liquids are similar to tablets but can be a good alternative if you have trouble swallowing tablets.

Antihistamine nasal sprays can help with symptoms of a runny or blocked nose.

When to take antihistamines

Many people find that their symptoms are worse in hay fever season, which in the UK is between late March and September, especially in warm weather. This is when the pollen count is highest, meaning there is more pollen in the air.

If possible, it is best to take antihistamines before you are exposed to pollen or another allergen. Antihistamines are most effective if taken before you come into contact with pollen and can stop a hay fever reaction entirely. However, even if antihistamines are taken after exposure to pollen, they can still reduce the severity of hay fever symptoms.

Antihistamine interactions

It’s always worth being cautious when taking any new medication. If you are unsure about how antihistamines may interact with another medication you are taking, you should talk to a doctor or pharmacist. Some of the medications that can interact with antihistamines are:

●      Antidepressants         

●      Indigestion medicines           

●      Cough and cold medicines that also contain antihistamines           

●      Sedatives or insomnia medication    

●      Opiate painkillers      

●      Seizure medications

You should also avoid alcohol when using an antihistamine. This is especially important if you are using a drowsy antihistamine, as alcohol may make you drowsier, which can lead to extreme fatigue or confusion.

Do antihistamines have side effects?

Like most medications, antihistamines can have side effects. Some people may experience different side effects from antihistamines and others might not have any. If you have side effects while taking one type of antihistamine, you may find that another type works better for you. If you are worried about side effects, you should speak to a pharmacist. Some of the more common side effects of antihistamines are:

●      Drowsiness – especially from drowsy antihistamines

●      Dry mouth

●      Headache and a feeling of tightness in the head

●      Blurred vision

●      Confusion – more commonly associated with drowsy antihistamines        

●      Poor coordination

●      Difficulty urinating     

●      Constipation

●      Nausea and feeling sick         

●      Dry eyes         

●      Dizziness        

●      Rapid heart rate

The chance of side effects may be increased if you are taking other medication or drink alcohol with antihistamines.

FAQs about antihistamines

How much antihistamine can I take in 24 hours?

Different antihistamines have different daily doses. You need to check the label and patient information leaflet to find out how many antihistamines you can take per day. Some antihistamines are designed to be taken once a day, while others should be taken throughout the day, as needed. Creams, eye drops, and liquid antihistamines will all have different doses too. If you are unsure how much you should be taking, a pharmacist can help.

Can you take antihistamines when pregnant?

It is always best to speak to a doctor about taking medications while you are pregnant or breastfeeding. The NHS advises that loratadine is often suitable while pregnant, but each pregnancy is different, so it is always best to check with a doctor or pharmacist. You can also take some antihistamine medications while breastfeeding. Again, your doctor or pharmacist will be able to recommend what’s best for you.

How long do antihistamines take to work?

Different types of antihistamines can take different lengths of time to work. Most tablet antihistamines should start to work within 30 minutes, but they are at their most effective after one or two hours. Antihistamines work best if you take them before you are exposed to pollen or another allergen. The patient information leaflet that comes with your chosen product will indicate when to take them to ensure they’re most effective.

What happens if you take too many antihistamines?

The effects of taking too many antihistamines are different depending on what type of antihistamine you are using. The patient information leaflet for your antihistamine will explain what could happen if you take too much. Common effects of taking too many antihistamines include dilated pupils, increased heart rate, and fever-like symptoms.

If you take too many drowsy antihistamines you may feel unusually tired, have difficulty concentrating or staying awake, and could have a decreased heart rate. If you are concerned that you have taken too many antihistamines, you should seek medical advice.

 Useful resources

https://www.nhs.uk/conditions/hay-fever/

https://www.nhs.uk/conditions/antihistamines/

https://www.nhs.uk/conditions/allergies/