The prefix anti- means against or opposed to. Thus, to understand what an antihistamine is one first needs to understand what histamine is.Histamine is a naturally occurring body chemical that acts as a chemical messenger in every part of the body. The message delivered depends upon the receptor in the various body sites. For example, in the skin, there is some something called an H-1 receptor (like a keyhole) onto which histamine (the key) attaches and delivers the message telling a skin blood vessel to dilate and allow the water part of blood to leak out into the tissue. A common example of this is what happens with a mosquito bite. When a mosquito injects its digestive juice into a person's skin in order to suck out blood, the body has an allergic reaction to the mosquito's juice. That allergic reaction releases histamine locally which dilates the blood vessels around the site of the mosquito bite. Thus that little white lump that appears immediately after a mosquito bite is mediated by histamine attaching to its H-1 receptor.

    Other examples of the function of H-1 receptors include the glands of the mouth and nose and eyes that secrete mucus that allow these areas to remain moist. When a person develops a cold, these areas become irritated and histamine is released to stimulate into the glands. The H-1 receptors in these glands then cause secretion of more mucus and one gets a runny nose characteristic of a cold.

    A third area in which histamine causes unpleasant symptoms is what happens with an allergic reaction. If a person is allergic to poison ivy or to penicillin or something such as cat hair, the allergic reaction can lead to the release of histamine in large amounts all over the body. This can cause potentially fatal problems such as a sudden onset of asthmatic attacks, spasm of the larynx stopping the air supply and severe sudden drops in blood pressure that can be incapacitating. Anyone who has ever had an acute allergic reaction remembers how horrible this can be. This reaction is mediated and caused by the large amounts of histamine released in an allergic response.

    As you might imagine, since histamine generally causes unpleasant symptoms, pharmaceutical companies began to look for products that would get in the way of an allergic reaction many years ago. The first products on the market came in 1940 and there followed a host of products that are now available over the counter. These include Benadryl, Dramamine, PBC, and Chlor-Trimeton. All of these products have the side effect of causing sedation but they all decrease the symptoms of an allergic reaction and are used as over-the-counter "cold" pills. They have the side effects of causing people to become sleepy, develop dry mouth and dry eyes and develop a more rapid heartbeat. In older individuals, they can cause urinary retention in men and constipation in men and women. Like all medications, antihistamines have beneficial effects and some side effects.

    In the last 10 to 15 years, there has come available a second generation of antihistamine products that are called nonsedating. Some of these are now available over the counter without a prescription. Several of them had dangerous heart related side effects and were removed from the market; Hismanal and Seldane are examples of drug monitoring by the FDA leading to removal from the market. Others have not had such dangerous side effects and are available by prescription through your local healthcare provider. The most prominent of these is Claritin.

    It turns out that the stomach and small intestine have a different kind of histamine receptor called an H-2 receptor. When histamine attaches to these receptors in the stomach, the stomach makes more acid as a primary effect. It also contracts somewhat more vigorously and can cause cramping. The use of a classical antihistamine such as Benadryl or Claritin will have no effect on the stomach H-2 receptor. Instead, Claritin and Benadryl act on the H-1 receptors on the bronchial system, the skin and the blood vessels. The pharmaceutical industry developed a drug that will block the effect of histamine on the stomach, and this family of drugs are called the H-2 blockers: Tagamet, Zantac, Pepcid and Axid. These are all commercial products and all are available over the counter now without a prescription. These drugs have the beneficial effect of decreasing the amount of acid that the stomach makes and relaxing the stomach muscles so that ulcer disease and the tendency to have ulcers is improved. These drugs cut down the amount of acid that the stomach makes and thereby decreases "acid upset stomach". For many years, these drugs were the mainstay of treatment for ulcers and do much better than no treatment at all.

    Like all drugs, there are side effect problems with some of the H-2 blockers. Specifically, Tagamet has a feminizing effect which women find useful but men can find very disturbing. Prolonged use of high-dose Tagamet can cause breast enlargement for men. Zantac, Pepcid and Axid do not have this side effect and are preferred products.

There is an additional use of the H-2 blockers in patients that have shingles. Shingles is an infection that occurs secondary to the chickenpox virus and can be very painful. Much of the pain from this condition is mediated by H-2 histamine receptors in the skin and the use of Tagamet has been shown to significantly decrease the pain from shingles. On the downside, it turns out that in the lungs, there are H-2 receptors which are bronchodilators when stimulated and tend to alleviate asthmatic symptoms. These are balancedby H-1 receptors that constrict the muscle in the bronchial tree and cause bronchospasm and asthmatic-like symptoms. While the effect is not substantial, there is a slight tendency for the H2 blockers to worsen the asthmatic and bronchitis-like symptoms.

    Lastly, there is a third type of histamine receptor (H-3) that seems to reside primarily in the brain. This is another of the body's balancing acts in that H-1 receptors in the brain tend to cause the person to become wakeful while H-3 receptor stimulation tends to cause the person to become sleepy. To this time, there are no medications that are directed at the H-3 receptor because we do not understand enough about it to manipulate it.

    With all of the above, it is seen that histamine and its receptors in various parts of the body form a very complex system. Many of the histamine effects on the body are objectionable and noxious. Thus, the use of medications as antihistamines is one of the major drug expenditures in our society. Using antihistamines to stop allergic reactions and hives and upper respiratory tract symptoms can be highly beneficial. These are balanced by the adverse side effects of dry mouth, dry eyes, difficulty for men to pass urine and sedative effects that come from some of the antihistamines. Note that the sedative effects are taken advantage of when many of the over-the-counter sleep preparations such as Tylenol PM, that has a small amount of Benadryl in it, to help a person go to sleep. Men that take this drug and get a good night's sleep may find themselves having a difficult time passing urine the next morning. The antihistamines are a clear example of Hoffsten's Second Law: Nothing is ever simple. The healthcare providers at your local clinic can be a significant help in advising on the use and provision of the antihistamine family of medications. Used correctly, they can be very beneficial. Used injudiciously, the side effects can be substantial.