Since colds aren't caused by allergy, however, the beneficial effect of antihistamines for the cold is less than for allergy. Like all medications, antihistamines have a few side effects. The most common one is that of sedation. Different preparations have different amounts of this side effect and some people react more to some than others. This side effect can have its uses.
Antihistamines make a good sleep aid for people with occasional insomnia.
In fact, most over the counter sleeping pills are just antihistamines. Another side effect is dry mouth. Because of these side effects, many people find it safer to take their first dose of a new antihistamine when being sleepy will not be too much of a problem. They should not be used if you plan to drive a long trip or taken if you will be drinking alcohol. This class of medications acts on the tiny muscular coating of blood vessels and causes constriction of the vessel.