Overview:
Antihistamines are medicines used to relieve allergy symptoms such as sneezing, runny nose, itchy eyes, and hives. They help with conditions like hay fever, pet allergies, insect bites, and allergic skin reactions. Some antihistamines can also help with travel sickness or difficulty sleeping.
How It Works:
When you come into contact with something you’re allergic to — like pollen or dust — your body releases a chemical called histamine. Histamine causes swelling, itching, and mucus production. Antihistamines work by blocking histamine’s effects, which reduces these allergy symptoms.
Mode of Action:
Antihistamines attach to H1 receptors (the spots where histamine normally acts) on cells throughout the body. By taking histamine’s place, they stop it from triggering the allergic response. There are two main types:
- Older antihistamines (like chlorphenamine or diphenhydramine) which can cause drowsiness.
- Newer antihistamines (like cetirizine, loratadine, and acrivastine) which are less likely to make you sleepy.
Common Side Effects:
- Sleepiness or drowsiness (mainly with older types)
- Dry mouth or dry eyes
- Headache
- Mild dizziness
Serious Side Effects:
- Fast or irregular heartbeat (rare)
- Severe allergic reaction (swelling of the face, lips, or throat — very rare)
- Confusion, especially in older adults using sedating antihistamines
- Difficulty urinating (rare, but can happen in men with prostate problems)
