Older, sedating antihistamines such as chlorphenamine and diphenhydramine cause drowsiness, and alcohol adds to this effect, leading to marked sleepiness, slowed reactions, and impaired judgement. This combination makes driving or operating machinery dangerous and can be especially risky for older adults.
Newer, less-sedating antihistamines like cetirizine, loratadine, and fexofenadine are less likely to interact strongly with alcohol, but some people still feel more drowsy when they mix the two, so it is sensible to see how the medicine affects you before drinking.
If you need an antihistamine and plan to drink, a non-drowsy option taken in the daytime is the safer route, and a pharmacist can advise. Avoid driving if you feel sedated, and seek care for severe drowsiness, confusion, or difficulty breathing.