Do antihistamines have any side effects?

Some antihistamines (both over-the-counter and prescription) can make you drowsy, slow your reaction time, and impair your judgment. The older "first-generation" antihistamines, which are more likely to cause such effects, include chlorpheniramine (Chlor-Trimeton, others), diphenhydramine (Benadryl), clemastine (Tavist), and brompheniramine (an ingredient in the combination product Dimetapp). The "second-generation" antihistamines have fewer side effects—in particular, drowsiness and nasal dryness—than the older ones. These "non-sedating" medicines include loratadine (Claritin, Alavert, others), available over the counter, and fexofenadine (Allegra), desloratadine (Clarinex), and azelastine (Astelin nasal spray), all available by prescription.

Donna Hill Howes, RN
Family Practitioner

Side effects may include abdominal pain, loss of appetite, nausea, vomiting, diarrhea, and constipation. Other side effects include dry mouth, nose, or throat, vision problems, ringing in the ears, drowsiness, dizziness, insomnia, tremor, nervousness, irritability, and weakness. Some people have trouble emptying the bladder. Men may have erection problems.

Most doctors and guidelines do not recommend using antihistamines to treat sleep problems for many reasons. They are less effective than prescription sleep drugs. They can cause problems with mental and physical function. The sleepiness they cause can carry over into the next day, so that even with more sleep, a person may not feel well-rested. Antihistamines actually cause insomnia in some people. 

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