Antihistamines are a type of medication commonly used to treat allergy symptoms such as runny nose, sneezing, and itchy eyes. One of the most well-known side effects of antihistamines is drowsiness, which can cause the user to feel sleepy or lethargic. This effect is particularly pronounced in older antihistamine medications, but can also occur with newer, non-sedating antihistamines.

The reason why antihistamines cause drowsiness is related to their mechanism of action. Histamine is a chemical that is released by the body in response to allergic reactions. It acts on the histamine receptors in the body, causing symptoms such as itching, sneezing, and watery eyes. Antihistamines work by blocking the histamine receptors, preventing histamine from binding to them and causing symptoms.

However, histamine receptors are not only found in the body’s allergic response system. They are also found in the brain, where they play a role in regulating arousal and wakefulness. When antihistamines bind to histamine receptors in the brain, they can cause drowsiness and sedation. This effect is more pronounced in older antihistamines, which are more likely to cross the blood-brain barrier and bind to histamine receptors in the brain.

In addition to their sedating effects, antihistamines can also cause other side effects such as dry mouth, constipation, and dizziness. These effects can be more pronounced in older adults, who may be more sensitive to the medication. It is important to consult with a doctor before taking antihistamines, and to follow the instructions on the medication label to avoid side effects and ensure safe use.