Antihistamines and decongestants are two of the main treatments for sinusitis--the inflammation of the mucous membranes lining the facial sinuses. Understanding the differences among these various medications can help you to use them most effectively and to avoid taking medicines you don't really need.
Histamine is an inflammatory chemical released by immune system cells, often triggered by an infection or allergy. It's the signal for mucus production in the nose and sinuses, which then causes most of the bothersome symptoms. Antihistamines block this signaling by histamine, thereby reducing future mucus production. Note, however, that antihistamines won't get rid of the mucus that's already there. That's what decongestants are for--literally to reduce the congestion.
There are many types of antihistamines from which to choose. One of the oldest and best known is diphenhydramine, which does not require a prescription. There are a few other, similar drugs, also in over-the-counter (OTC) cold or sinus tablets. Frequently, the name of the active ingredient in these medications ends with "amine," but there are others, too. This class of drug typically works for about 4 to 6 hours, and the most significant side effect is drowsiness.
Non-sedating (less likely to cause drowsiness) antihistamines last longer, usually 12 to 24 hours. The only downside to these products is that they tend to be somewhat weaker than the sedating ones. Loratidine (Claritin®, Alavert®) and cetirizine (Zyrtec®) are OTC, and others are available by prescription.
A few cautions: Most antihistamines can worsen certain types of glaucoma, so if you have this condition you may want to check with your doctor before using them. Antihistamines can also slow urine flow, especially in men with an enlarged prostate. Fortunately, these urinary side effects only last as long as you are taking the medication, and usually aren't severe enough to require medical attention. Unlike decongestants, antihistamines generally do not affect blood pressure and are safe to use even if you have high blood pressure.
Antihistamines can also be applied locally to the nose, in a spray, or to the eyes as eye drops. Such sprays and drops are great for people who can't tolerate or don't need the systemic antihistamines discussed above. They can also be used in addition to the systemic medicines by those who need a little additional, focused relief now and then. These local antihistamines are mostly available by prescription, so see your doctor if you're interested in them.
Some people only need an antihistamine occasionally, when their allergic symptoms are severe. If your symptoms are more bothersome, however, or plague you for prolonged periods, you may need to take an antihistamine on a regular schedule, multiple times daily, and many days in a row. And sometimes it makes sense just to plan for regular daily use to prevent the symptoms from becoming intolerable. Fortunately, such regular use is perfectly safe most of the time.