What Is the Difference Between an Antihistamine and a Decongestant?

Decongestants reduce swelling and relieve pressure, and antihistamines block the chemical in the body that causes itching and swelling in the nasal passages, explains WebMD. Decongestants are useful when swelling and pressure make it difficult to breathe through the nose, and antihistamines help stop runny noses and sneezing. Antihistamines may not work on runny noses due to a cold virus, as histamine is not the main source of the symptoms; however, in some cases, they may provide relief.

Decongestants and antihistamines are available over the counter and often in conjunction with each other. Many cold and allergy medicines combine them with pain relievers as well. Before using them, people should check the ingredients to avoid mixing similar medicines in unsafe amounts, warns WebMD. This is especially true in preparations that contain acetaminophen, which can cause liver damage in high doses.

The side effects of decongestants include a higher blood pressure, so patients with high blood pressure or heart disease should ask a doctor before taking them, urges WebMD. Decongestants may cause difficulty sleeping, so they work better as daytime medicines. Antihistamines tend to cause drowsiness, making them more suitable at night. Other common side effects of antihistamines are dryness in the mouth, dizziness and headache.