As the name suggests, antihistamines block the action of histamine. When someone has an allergy to a normally harmless substance like pollen or pet dander, the immune system treats it as if it were 'foreign' like a virus or bacterium. It responds to the perceived threat by unleashing a cascade of inflammatory molecules, one of which is histamine.

Frequently Asked Questions

What are antihistamines?

Antihistamines block the action of histamine. Histamine drives the characteristic symptoms of an allergic reaction, namely:

What are antihistamines used for?

Antihistamines are used to treat the symptoms of allergic conditions including:

  • Hay fever (also known as seasonal allergic rhinitis)
  • Non-seasonal (all year round) allergic rhinitis
  • Urticaria (nettle rash)
  • Atopic eczema
  • Wasp and bee stings; insect bites
  • Seasonal or perennial allergic conjunctivitis
  • Anaphylaxis (in conjunction with adrenaline)

Some antihistamines are also used to treat vertigo, travel sickness, and insomnia. Antihistamines cannot cure an allergy but may provide very significant relief from symptoms.

How do I choose, and use an antihistamine?

Many antihistamines are available over the counter. Higher doses may require a prescription. There are two types of antihistamine – first generation (sedating) and second generation (non-sedating). Make sure you know which kind you are using. Antihistamines come in different formulations: tablet, capsule, syrup, nasal spray, gel, cream and lotion.

What are the first generation antihistamines?

These are older drugs which are rather non-specific in their action. Their action on the brain means that they have a sedating effect. Hence they are sometimes also referred to as sedating histamines.

What are the second generation antihistamines?

The second generation, or non-sedating antihistamines, are more specific in their target and have little effect on the brain. Examples include:

Benadryl Allergy Relief, Benadryl Plus Capsules
Benadryl Allergy Oral Syrup, Benadryl for Children Allergy Solution, Benadryl One-a-Day Relief, Piriteze Allergy, Pollenshield Hayfever Relief, Zirtek Allergy Relief

Which ones are available for the eyes and nose?

To relieve runny eyes and nose there are various sprays and drops, including:

Zaditen, eye drops
Rhinoblast, nasal spray, Optilast

What antihistamines are available for the skin?

Skin allergies, like reactions to wasp and bee stings, can be treated with creams and lotions like Benadryl Skin Allergy Relief Cream and Wasp-Eze Bites and Springs Spray, Wasp-Eze.

Who should not take antihistamines?

Most people can safely take antihistamines. However, there are some exceptions, including:

  • Women who are pregnant, or breastfeeding
  • People with various health conditions which may be made worse by an antihistamine – these include: epilepsy, high blood pressure, liver or kidney disease, overactive thyroid, diabetes and asthma
  • People in the above two categories should consult their GP or pharmacist on whether it would be wise to take an antihistamine
  • Some antihistamines are not suitable for children. Always check the label.

Do antihistamines have any side effects?

Yes, so always read the patient information leaflet thoroughly and keep it for future reference. The main side effect of the sedating antihistamines is, as you might expect, drowsiness. Never drive or operate machinery when taking one of these drugs and be aware that the effect may persist for some time, or into the next day if you take a dose at night. There is some evidence that people who regularly take sedating antihistamines are more likely to have a serious accident. These drugs may also affect your mental performance at work or school so it is better to opt for a non-sedating antihistamine if this is important to you, particularly if you have an important event like an exam or wedding coming up. You should also avoid alcohol if you are on a sedating antihistamine as they potentiate the effect of alcohol on the nervous system. Half a pint of beer will have the same effects as two or three pints and you might drive as if you were over the alcohol limit. In short, do not underestimate the sedating effect of a first-generation antihistamine. Driving simulation and psychometric tests have shown that the sedative effect, with slowed reaction times, is present even when people are not aware of feeling drowsy. It is best to opt for a non-sedating antihistamine wherever possible.

Less common side effects of antihistamines include:

  • Nausea
  • Dry mouth
  • Blurred vision
  • Headache

Rare side effects include dizziness, tremor, and low blood pressure. In general, the second generation antihistamines have fewer side effects than the older drugs (although some people still find they cause drowsiness). Children and the elderly are more likely to experience side effects from antihistamines.

Also, be aware of the potential for interaction between antihistamines and any other medicines you are on. The antihistamine mizolastine (Mizollen - prescription only) can interact with some medicines to cause abnormal heart rhythms. Some cough and cold remedies contain an antihistamine, so don't take these with an antihistamine medicine for allergy as you may end up taking too much antihistamine overall. If you have further questions, talk with your pharmacist.

Is there any research going into antihistamines?

Most effort is going into the search to find a vaccine against allergic disease and to find drugs that target parts of the immune system other than histamine. However, there could be some potential in looking for more specific drugs that target just the histamine receptors involved in allergy. At present, four different histamine receptors are known:

  • H1 – which causes inflammation when histamine binds it and which is hence the target of the antihistamines used to treat allergy
  • H2 – stimulates the production of stomach acid and the target of antihistamines used to treat stomach ulcer
  • H3 – the receptors in the brain; when antihistamines bind these, they cause sedation
  • H4 – recently discovered and its role is not clear, though it appears to be involved in the immune system

Further research into these receptors may produce antihistamines with different medical applications and maybe even one which is extremely specific to H1 and may have fewer side effects than current drugs.

1 of 3