What is a common side effect of 1st generation antihistamines like diphenhydramine?

• A. Increased energy
• B. Dry mouth
• C. Anxiety
• D. Headache

Answer: (B)

First-generation antihistamines, such as diphenhydramine, are known for their effectiveness in alleviating allergy symptoms, but they also have notable side effects due to their ability to cross the blood-brain barrier and affect the central nervous system. One common side effect is dry mouth, which occurs because these medications block the action of acetylcholine, a neurotransmitter that stimulates salivary gland secretion. This anticholinergic effect results in reduced saliva production, leading to the sensation of dry mouth.

In contrast, the other options do not typically align with the pharmacological effects of first-generation antihistamines. Increased energy is not a characteristic side effect, as these medications can often cause sedation or drowsiness. Anxiety is not associated with first-generation antihistamines; instead, sedation is more common. Similarly, while headaches can be a side effect, they are not as consistently reported as dry mouth following the use of these medications. Thus, the prominent effect of dry mouth is clearly linked to the mechanism through which first-generation antihistamines operate.