Antihistamines (Antiallergic) Important Information (Uses, Types, Safety in pregnancy)

By | October 26, 2017

Antihistamines are a group of medicines which act by blocking the action of the chemical called histamine in the body. Either H1 or H2 histamine receptors can be blocked by medicines, but the group commonly known as antihistamines blocks the H1 receptor. They have a number of uses, but are most often used to treat allergies.

What are antihistamines used for?

If you are one of the many people who get hay fever each year, you will know all about antihistamines. This is one of the most common uses of antihistamines. They can also be used for various other problems. For example:

  • Hives (urticaria).
  • Itching (pruritus).
  • Feeling sick (nausea) and being sick (vomiting).
  • Severe allergic reactions (anaphylaxis).

Where can I get them?

Some of these medicines are available to buy over the counter from your pharmacist. Others are only available on prescription.

How do I take them?

These medicines come in a variety of forms, as mentioned above. Your doctor or pharmacist will advise you on how to take your medication, including what dose and how often. Read the leaflet that comes with your particular brand for further information.

How quickly do antihistamines work?

An antihistamine tablet typically starts to work within 30 minutes after being taken. The peak of effectiveness is typically within 1-2 hours after being taken.

How long is treatment needed?

This can vary depending on the reason for treating you. If you have hay fever you may take the medicine throughout the pollen season.

How effective are they?

Usually it is possible to find an antihistamine which keeps your symptoms controlled. Sometimes you may need to experiment and try more than one to find the one that works best for you. For hay fever, sometimes if the antihistamine tablet isn’t enough by itself, you may need eye drops and/or a nasal spray as well to completely sort out all your symptoms.

Antihistamines are generally more effective when taken constantly rather than intermittently. This is particularly applicable for people with hay fever (seasonal allergic rhinitis). In the summer months the pollen count is generally higher and you may be in contact with the allergen on a regular basis. Taking the medication regularly may help keep your symptoms under control. Their effectiveness will also depend on the dose you take and what form the medicine is given in.

Can I take them if I am pregnant or breast-feeding?

Drug companies are naturally pretty reluctant to go around testing medicines on pregnant or breast-feeding women, so there aren’t any studies to guide them. They tend to advise against to be on the safe side, although there is no evidence they cause a problem. Discuss this with your doctor who will talk you through the options. If the benefits of treatment are thought to outweigh any possible risks, the one usually advised is loratadine.

What conditions are antihistamines used to treat?

Antihistamines are commonly used:

  • To relieve the symptoms associated with hay fever (seasonal allergic rhinitis). These can include:
    • Inflammation of the nose and eyes (rhinitisand conjunctivitis).
    • Itching of the eyes, nose and throat.
    • Runny nose (rhinorrhoea).
  • To reduce the severity of the rash and itching associated with nettle-type rashes such as hives (urticaria)and generalised itching (pruritus).
  • To help with the rash and itching following insect stings or bites.
  • To prevent motion sicknessand other causes of feeling sick (nausea).
  • Occasionally to treat severe morning sickness in pregnancy.
  • In the care of the terminally ill, for their sedating and antisickness effects.
  • In the emergency treatment of severe allergic reactions.

Are there different types of antihistamines?

Generally, antihistamines have been classified into two groups:

  • First-generation or sedating antihistaminescan cause significant drowsiness and are generally more associated with the antimuscarinic side-effects mentioned above. These include alimemazine, chlorphenamine, clemastine, cyproheptadine, hydroxyzine, ketotifen and promethazine. These medicines may be used for their sedative effects should your sleep be disturbed by itching.
  • Non-sedating or second-generation antihistaminesare newer medicines which generally cause less drowsiness. However, anyone taking these medicines while performing skilled tasks – for example, driving – should be aware that a sedative effect may still occur and, in particular, in combination with alcohol. Second-generation antihistamines include acrivastine, cetirizine, desloratadine, fexofenadine, levocetirizine and loratadine.

Which is the best one?

All antihistamines work pretty well to reduce symptoms of allergy. Your doctor or pharmacist may advise or prescribe a particular antihistamine depending on the cause of your allergy and on whether you require a sedating or non-sedating medicine. For example:

  • In general, antihistamines are probably roughly equally effective in reducing the symptoms of hay fever (seasonal allergic rhinitis) and hives (urticaria). However, non-sedating antihistamines tend to be used more commonly as they cause less drowsiness.
  • Second-generation antihistamines are generally advised for most allergic situations as they cause less drowsiness.
  • Cetirizine, fexofenadine, or loratadine are often recommended for urticaria.
  • Antihistamine eye drops may be advised when itchy eyes are a particular problem – for example, azelastine eye dropsor ketotifen drops.
  • A sedating antihistamine may be particularly helpful at bedtime for children who have allergic symptoms. Cough medicines containing sedating antihistamines are not suitable for children under the age of 6 years, and a pharmacist’s advice is needed for children between the ages of 6 and 12 years.

For other conditions, specific antihistamines may be used. For example, cyclizine and promethazine teoclate are used for feeling sick (nausea) and being sick (vomiting), not for hay fever. Chlorphenamine is the antihistamine most used in an emergency situation such as anaphylaxis, and may be given by injection in this situation. Diphenhydramine (Nytol®) is sold over the counter as a sleep remedy.

What preparations are there?


Antihistamines come as:

  • Tablets (most commonly).
  • Liquid medicine (for those who can’t swallow tablets – for example, children).
  • Injections (for serious allergic reactionswhere immediate treatment is necessary).
  • In eye drops.
  • In nose drops and spray.
  • In creams and ointments.

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