Antihistamines are used by thousands of people each day; they help to ease symptoms associated with conditions such as hayfever and allergies, but can also be used to treat atopic eczema, insomnia and allergic conjunctivitis.

How do antihistamines work?

Antihistamines work by blocking the effects of a protein called histamine; histamine is released by the body when the immune system detects a harmful foreign object (usually a virus or bacteria) as it helps to protect the body against illness. Although histamine is a useful protein, it can be released unnecessarily when the body mistakes a harmless substance, such as dust or pollen, for a harmful foreign object; this is what happens when people have allergic reactions. When histamine is released it causes the skin to become swollen as a result of the vessels underneath the skin dilating; this is why the skin often becomes inflamed during an allergic reaction. Histamine can also cause the skin to become itchy. In addition, when the vessels in the lungs become swollen, this can also cause sneezing, coughing and difficulties with breathing; this happens because the airways become narrower.

Are antihistamines safe for everyone?

Most people can take antihistamines but people with certain health conditions may not be able to take them; examples of these conditions include:

  • Heart disease
  • Epilepsy
  • Diabetes
  • Hypertension (high blood pressure)
  • Liver or kidney disease
  • An overactive thyroid gland
  • Glaucoma
  • An enlarged prostate gland

Pregnant women should consult their GP before taking antihistamines as some medicines may not be suitable. Some medicines are also not suitable for children aged 2 or under; you should consult your doctor or pharmacist before giving your child antihistamine.

Are there any side-effects?

Some people experience side-effects with antihistamine medication and some people don’t’examples of side-effects include:

  • Drowsiness
  • Dizziness
  • Dry mouth
  • Inability to sleep
  • Itchy skin
  • Hallucinations

Rare reactions include tightness in the chest and a racing heartbeat.

Types of antihistamine

Antihistamines are classified as first or second generation antihistamines; this refers to when they were developed; first generation antihistamines were developed after the 2nd World War, while second generation medicines were developed during the 1990’s. As a general rule, second generation medicines are now recommended to patients, but first generation medicines may be more suitable for some patients. Examples of common antihistamines include loratidine and ceterizine.

Many antihistamines are now available over the counter but you should always read the label carefully and don’t assume you can take them just because they are available without a prescription; this is particularly important if you are on other medication or have an underlying health problem. Always ask your GP or pharmacist if you are unsure. Antihistamines are now available in several different forms, including tablets, nasal sprays, eye drops and creams.

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