What side effect is commonly associated with antihistamines?

Antihistamines are primarily used to alleviate allergic reactions by blocking the action of histamine, a compound that is responsible for many allergy symptoms. A well-known side effect of antihistamines is drowsiness. This occurs because many antihistamines can cross the blood-brain barrier and have a sedative effect on the central nervous system. As a result, individuals taking these medications may experience an increased sense of sleepiness or sedation.

In terms of the other options, while some antihistamines can lead to side effects such as nausea, weight gain, and gastrointestinal issues, drowsiness remains the most prevalent and widely recognized side effect. This is particularly relevant for first-generation antihistamines, which are more likely to cause sedation compared to second-generation options, which have been formulated to reduce this side effect while still providing allergy relief.