The differences between an antihistamine and a decongestant are significant, as each is a completely different class of drugs. Decongestants help reduce nasal congestion by narrowing nasal blood vessels, which reduces swelling and makes it easier for a decongestant user to breathe. Antihistamines, on the other hand, have multiple uses. For example, antihistamines can help minimize allergy symptoms, assist people with relaxation and sleep, and curb nausea. Many cold and allergy medication formulas, both over-the-counter and prescription, combine an antihistamine and a decongestant.
Decongestants are often used by individuals who suffer from sinus congestion, sinus infections, or post-nasal drip. As decongestants reduce swelling of the nasal passages, users can get relief from cold and allergy symptoms and can facilitate recovery from sinus infections. Two decongestants, phenylephrine and pseudoephedrine, are commonly included in over-the-counter medications, though because of a concern about their use in methamphetamine production, some areas require a doctor's prescription for products containing pseudoephedrine. Decongestants are available as both a pill and as a nasal spray.
Antihistamines are used to treat a variety of conditions, though not all antihistamines have the same versatility. Older antihistamines, such as chlorpheniramine, are known to cause drowsiness. While this can make them undesirable as daytime medications, many people use antihistamines as a temporary treatment for insomnia. Antihistamines are also used to treat motion sickness and to control vomiting.
While they can be used to treat a number of conditions, antihistamines are probably best known for their ability to treat allergies. When the body is exposed to substances to which it is allergic, it produces histamines, chemicals that can cause redness and swelling in the body, often on the skin and in the eyes and nose. In some cases, colds can also cause a release of histamines, which is why an antihistamine may help to relieve cold symptoms. When cold or allergy sufferers use both an antihistamine and a decongestant together, they may experience greater relief than if they use just one type of drug.
Individuals using drugs should consult with their pharmacist or doctor over their correct use. Decongestants, for example, become less effective over time and can actually contribute to nasal swelling with overuse. Some antihistamines work better at treating specific conditions than others. For example, some of the newer antihistamines do not effectively treat runny nose caused by a cold, but are more effective at treating nasal allergies. In all cases, users of these products should be aware of potential side effects, though some believe that a formula containing both an antihistamine and a decongestant can minimize the more common side effects of both drugs.