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 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:
- headache
- dry mouth
- dry nose
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.
Third-generation antihistamines
Third-generation antihistamines have some similar side effects to second-generation antihistamines, including:
- drowsiness, although this is less common than with first-generation antihistamines
- headache
- dry mouth
- feeling sick
However, third-generation antihistamines have been found not to have the same risk of heart problems.
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:
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