Antihistamines are commonly used to relieve allergy symptoms, but they can cause side effects. Some people may experience drowsiness, dry mouth, or dizziness. These effects occur because antihistamines block the action of histamine, a substance in the body that contributes to allergic reactions. In addition to drowsiness, some antihistamines can lead to difficulty urinating or blurred vision. Older adults may be more sensitive to these side effects, increasing the risk of confusion or falls. It's important to consult a healthcare provider if side effects are severe or persistent.