They offer relief of nasal allergies by blocking the effects of histamine. But histamine release is only one of many mechanisms that cause symptoms of nasal allergies. Antihistamines may relieve your runny nose, but they probably will not relieve your stuffy nose. Some antihistamines can be purchased without a prescription. Others must be prescribed by a doctor.
Drowsiness is a common side effect of many antihistamines.
Decongestants are also commonly used to treat nasal allergies. They relieve nasal congestion by shrinking blood vessels in the nose. Like antihistamines, they are available with or without a prescription. Oral decongestants may cause irritability and sleeplessness in some people. When decongestant sprays are used too often or for too long, rebound congestion can occur.