What is a common side effect of antihistamines?

Drowsiness or sedation is a well-known side effect of antihistamines, particularly the first-generation agents. These medications, which are often used to relieve symptoms of allergies, cold, and hay fever, cross the blood-brain barrier and can affect the central nervous system. This can lead to sedation, making individuals feel sleepy or lethargic after taking these drugs.

While some antihistamines are designed to be non-sedating (primarily second-generation antihistamines), the sedative effect remains a common concern for patients using first-generation options, such as diphenhydramine (Benadryl). Understanding this side effect is crucial, as it can influence a patient's daily activities, particularly if they need to drive or operate machinery.

The other options, while they may occur with various medications, are not as universally associated with antihistamines as drowsiness is. For instance, dry mouth can occur but is not primarily linked with antihistamines compared to sedation. Similarly, increased heart rate and gastrointestinal upset might occur due to other factors or medications but aren't typical side effects of antihistamines.