Antihistamines are used primarily to counter the effects of histamine, a neurochemical involved in allergic reactions. They also can reduce anxiety through their sedative effects and are sometimes used to treat insomnia. If you’ve ever taken a Benadryl (diphenhydramine), you know what I mean. Although not FDA-approved for insomnia, antihistamines promote drowsiness which in turn can influence sleep induction. They will not help with sleep maintenance, nor will they prevent early morning awakenings.
Unlike the benzodiazepines, antihistamines – over-the-counter or prescription – are not associated with a risk of dependency. They typically work within 20-30 minutes and are effective for four to six hours at the most. Although not habit forming, tolerance can develop to their sedative and anxiolytic effects requiring dosage increases. This can lead to grogginess, confusion, possible memory deficits and impaired performance. There are no credible studies on the long-term use of these drugs, even though they are routinely employed by people for long period of time.