Like all medicines, antihistamines can have side effects. Generally, these are more significant with first-generation antihistamines.

For a full list of specific side effects of your medicine, see the information leaflet that comes with your medication.

Most information leaflets can also be found online on the Medicines and Healthcare products Regulatory Agency (MHRA) or electronic Medicines Compendium (eMC) websites.

First-generation antihistamines

Common side effects of first-generation antihistamines include:

• drowsiness
• impaired thinking
• dry mouth
• dizziness
• constipation
• blurred vision
• an inability to fully empty the bladder (urinary retention)

It's important not to underestimate the effects of antihistamine-related drowsiness. Some first-generation antihistamines can impair co-ordination, reaction times and judgement in the same way that alcohol consumption can. Therefore you shouldn't drive or use power tools or heavy machinery after taking a first-generation antihistamine.

Less common side effects of first-generation antihistamines include:

• insomnia (difficulty sleeping)
• nightmares
• hallucinations (seeing or hearing things that aren't real)
• itchy skin

Rare side effects of first-generation antihistamines include:

• rapid heartbeat
• chest tightness

Contact your GP if you experience these rare side effects.

Side effects in children

Children have a greater risk of side effects from first-generation antihistamines, particularly drowsiness and impaired thinking.

Second- or third-generation antihistamines

Second- or third-generation antihistamines are less likely to cause drowsiness. If you find yourself feeling drowsy, don't drive, drink alcohol, or use tools or machines.

Other side effects of second- or third-generation antihistamines include:

• headache
• dry mouth
• dry nose
• feeling sick

These side effects don't usually last long and should pass quickly.

Rarer side effects include:

• rapid heartbeat
• chest tightness

However, second- and third-generation antihistamines have been found to have less risk of heart problems than first-generation antihistamines.

Contact your GP if you experience these rare side effects.

H2 receptor antagonists

Antihistamines used to treat stomach ulcers are known as H2 receptor antagonists. Side effects of this type of antihistamine are uncommon but may include:

• diarrhoea
• headache
• dizziness
• skin rashes
• tiredness

Yellow Card Scheme

The Yellow Card Scheme allows you to report suspected side effects from any type of medicine you're taking.

It's run by a medicines safety watchdog called the Medicines and Healthcare products Regulatory Agency (MHRA).

See the Yellow Card Scheme website for more information.