Which side effect is associated with the anticholinergic effects of antihistamines?

• A. Increased heart rate
• B. Headache
• C. Nausea
• D. Excess sweating

Answer: (A)

Antihistamines, particularly those with anticholinergic properties, can lead to an increased heart rate as a side effect. This occurs because anticholinergic agents block the action of acetylcholine, a neurotransmitter that plays a critical role in the parasympathetic nervous system, which is responsible for slowing heart rate among other functions. When the effect of acetylcholine is inhibited, the balance tilts towards sympathetic dominance, often resulting in elevated heart rate, known as tachycardia.

In contrast, other choices do not have a direct correlation with the anticholinergic effects typical of many antihistamines. Headaches may occur due to various non-specific reasons, and nausea could stem from different pharmacological effects but is not specifically tied to anticholinergic activity. Excess sweating is typically associated with other underlying conditions or medications that affect the autonomic nervous system differently. Understanding the specific mechanism of action for anticholinergic effects helps clarify why increased heart rate is the most relevant side effect in this context.