Commonest side effect of antihistaminics is?
**Core Concept**
Antihistamines are a class of medications used to treat various conditions, including allergies and itching. They work by blocking the action of histamine, a chemical involved in allergic reactions, on histamine receptors in the body.
**Why the Correct Answer is Right**
The correct answer is sedation, which is the most common side effect of antihistamines. This occurs because many antihistamines have a central nervous system depressant effect, which can cause drowsiness, fatigue, and impaired cognitive function. This is due to the ability of these medications to cross the blood-brain barrier and interact with receptors in the brain, such as the H1 receptor.
**Why Each Wrong Option is Incorrect**
**Option A:** Dry mouth is a possible side effect of antihistamines, but it is not the most common.
**Option B:** Urinary retention is a rare side effect of antihistamines, particularly in older adults.
**Option C:** Increased heart rate is not a common side effect of antihistamines, as most of them do not have a significant effect on the cardiovascular system.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the sedative effects of antihistamines can be more pronounced in older adults, individuals with liver or kidney disease, and those taking other sedating medications.
**Correct Answer: D. Sedation.**