Antihistamines
The difference between sedating and non-sedating antihistamines, and when each is appropriate.
Last revised
Medicines that block histamine, the chemical driving the itching, sneezing, runny nose and weals of an allergic response. The main practical distinction is whether they cross into the brain and cause drowsiness.
Two generations
Older, first-generation antihistamines such as chlorphenamine and promethazine cross the blood-brain barrier readily and cause marked sedation. Newer, second-generation agents such as cetirizine, loratadine and fexofenadine do so much less and are preferred for daytime use. Cetirizine is mildly sedating in a minority; fexofenadine is the least sedating of the common options.
Sedation is not a benefit
Sedating antihistamines are sometimes taken deliberately as sleep aids. They are not recommended for this: tolerance to the sedative effect develops quickly, sleep quality is altered, and next-day impairment of driving and concentration is well documented. In older adults, first-generation antihistamines carry anticholinergic effects associated with confusion, falls and urinary retention, and are generally best avoided.
Using them for hay fever
For seasonal allergic rhinitis, a non-sedating antihistamine taken regularly through the season works better than taking it only when symptoms flare. Where nasal symptoms dominate, an intranasal corticosteroid spray is more effective than antihistamines and is often the better first choice, provided it is used correctly and consistently for a couple of weeks before judging it.
What they do not treat
Antihistamines do not relieve the wheeze or breathlessness of asthma and are not a treatment for anaphylaxis. In anaphylaxis, adrenaline given by intramuscular injection is the only treatment that works, and antihistamines must never delay it. This is one of the most dangerous misconceptions in common circulation.
At a glance
| Term | Detail |
|---|---|
| First generation | chlorphenamine, promethazine; markedly sedating |
| Second generation | cetirizine, loratadine, fexofenadine; much less sedating |
| Least sedating common option | fexofenadine |
| Best for hay fever with nasal symptoms | intranasal corticosteroid spray |
| Nasal spray trial period | use consistently for about 2 weeks before judging |
| Avoid in older adults | first-generation agents, for anticholinergic effects |
| Treatment for anaphylaxis | intramuscular adrenaline, never antihistamines |
When to get help
Call 999 immediately for swelling of the lips, tongue or throat, difficulty breathing or swallowing, a hoarse voice, widespread rash with faintness, or collapse — this is anaphylaxis. Use an adrenaline auto-injector straight away if one is prescribed, lie the person flat with legs raised, and give a second dose after five minutes if there is no improvement. Do not rely on antihistamines.
Where to go next
Where this comes from
- NICE - Allergic rhinitis clinical knowledge summary
- NHS - Antihistamines
- Resuscitation Council UK - anaphylaxis guidance
Medical disclaimer
This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to a GP, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 999.
