All about antihistamines

Antihistamines are most commonly used to treat allergic reactions such as hay fever.

Other uses include the treatment of motion sickness, vertigo and sleeplessness. Availability and use of medicines may vary from country to country.

Why would I take antihistamines?

These medicines can be used to treat a range of conditions. One reason you would take antihistamines is to ease allergic reactions. These include:

• hay fever (allergic rhinitis)

• allergic skin conditions including urticaria (hives) and dermatitis

• itching

• insect bites or stings

You can also take some forms of antihistamine to help relieve symptoms of motion sickness, vertigo, and sleeplessness.

What are the main types of antihistamines?

There are two main types of antihistamine – the older, sedating form and the newer, non-sedating form.

The sedating form of antihistamine works in the brain as well as the rest of the body and can make you sleepy. An example is chlorphenamine maleate.

The non-sedating form doesn’t enter the brain so easily and won’t make you so sleepy. This type of antihistamine doesn’t relieve the symptom of feeling sick. Examples are acrivastine and loratadine.

How do antihistamines work?

Your immune system protects you from harmful substances such as bacteria and viruses. It does this by producing antibodies to harmful substances and removing these from your body.

In an allergic reaction, your immune system reacts to a harmless substance such as pollen because it mistakes it for a harmful substance.

One of the chemicals released by your immune system is histamine. It helps damaged tissue to heal but it also causes inflammation, which leads to symptoms such as:

• runny nose
• itchy eyes
• sore throat
• sneezing
• red, itchy skin

Antihistamines work by blocking histamine receptors in the body’s cells. This helps to prevent inflammation and eases allergic reactions.

In a more severe allergic reaction, other symptoms may include:

• difficulty breathing
• swollen lips and eyelids
• fast heartbeat

This type of allergic reaction is called anaphylaxis and although it can be helped by taking antihistamines, you should seek urgent medical attention as it can be fatal.

How to take antihistamines

Many antihistamines can be bought from a chemist or drugstore without a prescription.

Examples include chlorphenamine maleate, loratadine, and cetirizine. They are intended only for short-term use and for higher doses you will need a prescription from a doctor.

For hay fever, you can take antihistamines as tablets, capsules, or syrups that can all be swallowed, or use a nasal spray.

For some allergic skin conditions, or after an insect bite, you can rub gels or creams into the areas of your skin where there is a rash or itchiness.

Antihistamine creams and gels can sometimes cause an allergic reaction themselves, so you shouldn’t use them for more than three days at a time.

Special care

If you’re pregnant, it’s recommended that you don’t take antihistamines. This is also true if you’re breastfeeding because antihistamines can get into your breast milk.

If you feel you need to be treated with antihistamines while pregnant or breastfeeding, see your doctor for advice.

Side-effects of antihistamines

This section does not include every possible side-effect of antihistamines. Always read the patient leaflet that comes with your medicine and ask your pharmacist for advice.

There are two different forms of antihistamine – sedating and non-sedating. The sedating form has a side-effect of making most people feel very sleepy and can affect your coordination in a similar way to drinking alcohol.

This means it’s more dangerous to drive if you have taken a sedating antihistamine, even the next day after taking it the night before. An example of a sedating antihistamine is a chlorphenamine maleate.

With the newer, non-sedating antihistamines sleepiness is less of a problem. Side-effects that are more common with the older, sedating antihistamines include:

• headache
• difficulty in passing urine
• dry mouth
• blurred vision
• feeling sick or vomiting

Other, rare side effects of antihistamines include:

• low blood pressure
• abnormal heart rhythm
• dizziness
• confusion
• depression
• disturbed sleep
• tremor (a slight shaking of a part of the body, often the hands)
• fits
• allergic reactions (including swelling, rashes, breathing difficulties)
• blood or liver problems
• over-excitement in children

Children and adults over 65 are more likely to get side effects.

Interactions of antihistamines with other medicines

Check with your doctor or pharmacist before you take any other medicines or herbal remedies at the same time as antihistamines.

The antihistamine mizolastine can interact with some other medicines to cause a serious abnormal heart rhythm. This medicine is only available on prescription from your doctor, who will give you advice if you need to take it.

It’s best not to drink alcohol while taking antihistamines as this will increase the drowsiness side-effect.

Names of common antihistamines

Below is a list of common antihistamine medicines. These may also be known by different brand names in different countries.

Non-sedating antihistamines

• acrivastine
• cetirizine hydrochloride
• desloratadine
• fexofenadine hydrochloride
• levocetirizine hydrochloride
• loratadine
• mizolastine
• Sedating antihistamines
• alimemazine tartrate
• chlorphenamine maleate
• clemastine
• cyproheptadine hydrochloride
• hydroxyzine hydrochloride
• ketotifen
• promethazine hydrochloride
• For the eyes and nose
• antazoline (with xylometazoline)
• azelastine
• epinastine
• ketotifen
• promethazine hydrochloride
• Creams and lotions for the skin
• diphenhydramine hydrochloride
• mepyramine maleate
• Travel sickness and vertigo/nausea
• meclozine hydrochloride
• cyclizine
• cinnarizine
• Sleep aids
• diphenhydramine hydrochloride
• promethazine hydrochloride