What is a common side effect of diphenhydramine?

Diphenhydramine is an antihistamine that is commonly used to alleviate symptoms of allergies, such as sneezing, runny nose, and itchy eyes. One of its well-known side effects is drowsiness, which occurs because diphenhydramine can cross the blood-brain barrier and exert a sedative effect on the central nervous system. This property makes it effective as a sleep aid as well, but it also means that individuals taking the medication may experience significant drowsiness or fatigue.

This sedative effect is a result of its anticholinergic properties, which can inhibit neurotransmitters that are involved in wakefulness. Drowsiness can pose risks, especially when driving or performing tasks that require alertness, which is why caution is advised when using this medication.

Other side effects can occur, but drowsiness is particularly notable and often the primary reason why people may be cautious about taking diphenhydramine, especially in the daytime or before engaging in activities requiring full attention.