What is a common side effect of antihistamine use?

Antihistamines, commonly used to treat allergies, work by blocking the action of histamine in the body. A prevalent side effect of these medications is dry mouth, which occurs because antihistamines can inhibit the secretion of saliva. This effect is particularly notable with first-generation antihistamines, which can also cross the blood-brain barrier and lead to additional central nervous system effects, such as sedation. The drying effect on mucous membranes extends beyond just the mouth and can sometimes lead to discomfort.

While nausea, diarrhea, and increased appetite can occur with various medications or conditions, they are not typical side effects specifically associated with antihistamines. Therefore, dry mouth is the most common and recognizable side effect linked to their usage, making it the correct answer.