First-generation antihistamines
Common side effects of first-generation antihistamines include:
- drowsiness
- impaired thinking
- dry mouth
- dizziness
It's important not to underestimate the effects of antihistamine-related drowsiness. Some first-generation antihistamines can impair abilities such as co-ordination, reaction times and judgment in the same way that alcohol consumption can.
Therefore it's very important that you do not 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.
Second-generation antihistamines
A few people will experience drowsiness after taking second-generation antihistamines. If you find yourself feeling drowsy, do not drive, drink alcohol or use tools or machines.
As well as drowsiness, other side effects of second-generation antihistamines include:
These side effects are usually short-lasting and should pass quickly.
Rarer side effects include:
- rapid heartbeat
- chest tightness
Contact your GP if you have these rare side effects.
H2 receptor antagonists
Antihistamines that are used to treat stomach ulcers are known as H2 receptor antagonists. Side effects of this type of antihistamine are uncommon but may include:
