The most annoying side effect of OTC antihistamines is drowsiness. Though desirable at bedtime, it is a nuisance to many people who need to use antihistamines in the daytime. To some people, it is even hazardous. These drugs are not recommended for daytime use for people who may be driving an automobile or operating equipment that could be dangerous. Newer non-sedating antihistamines, available by prescription only, do not have this effect. The first few doses cause the most sleepiness; subsequent doses are usually less troublesome.