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People who take antihistamines for their side effects rather than for their intended purpose are following a potentially dangerous path.

More than a billion doses of medications containing antihistamines are consumed in the United States each year. They are sold as cold remedies, sleep aids, allergy relievers and tranquilizers."Most antihistamines are used safely and effectively to relieve the symptoms of colds and allergies," said Samuel Uretsky, a pharmacist at New York University Medical Center. "However, some people misuse them in an attempt to achieve a variety of responses, and some people become confused because of the way the products are marketed. Either way, you can get into trouble."

Because of legal restrictions on the sale of almost all sedatives and stimulants, some people turn to antihistamines for their side effects.

"Cold remedies are readily available legal drugs that cause central nervous system sedation or stimulation. They can be and are misused," Uretsky said.

In order to achieve a desired degree of sedation, people must often take larger doses of antihistamines than are generally considered safe, Uretsky said. In a single overdose, antihistamines can cause nausea, dizziness, and impaired reaction and muscular coordination.

Although serious side effects are rare, antihistamines have been associated with urinary retention, blurred vision and anemia. The drugs may be particularly dangerous in people with glaucoma, peptic ulcer or enlarged prostate.

Generally, antihistamines are meant to be taken for seven to 10 days, not indefinitely: if symptoms do not respond in that time, an underlying cause may be responsible and evaluation may be needed.

The placebo effect is also a factor. Given a placebo - an ineffective agent, such as a sugar pill - about 10 percent of the population will show an expected subjective response, like drowsiness. "Drugs tend to live up to the expectations held of them," Uretsky said.

Although there is no physical dependence associated with the use of antihistamines, a psychological dependence can lead to habitual use.

Antihistamines counteract the effects of histamine, a body chemical involved in allergic reactions.

Antihistamines can be effective in helping to relieve such cold and allergy symptoms as runny nose, itchy eyes and sneezing.

Many cold remedies are combinations of antihistamines and decongestants, which relieve nasal stuffiness. Decongestants cause a narrowing of blood vessels and relaxation of bronchial passages.