Which of the following is a common side effect of antihistamines?

Drowsiness is a well-known side effect of antihistamines due to their sedative properties. Antihistamines work by blocking the action of histamine, a substance in the body that causes allergic symptoms. Many first-generation antihistamines, such as diphenhydramine (Benadryl), are able to cross the blood-brain barrier and have a sedating effect, making drowsiness a notable adverse reaction.

While some antihistamines are designed to be less sedating and are referred to as second-generation antihistamines, they can still cause drowsiness in some individuals, particularly if taken in higher doses or in sensitive populations. This side effect is significant as it can impact daily activities, including driving and operating machinery, so it's important for patients to be aware of this potential outcome when using antihistamines.

In contrast, the other options generally do not align with the common side effects of antihistamines. Increased appetite and weight loss are not typically associated with these medications, and diarrhea is not a common effect, making drowsiness the most relevant side effect in the context of antihistamines.