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For best effects, people who have seasonal allergies should take antihistamines before allergy season starts or immediately after being exposed to an allergen. Even then, however, antihistamines do not cure allergies or prevent histamine from being released. They also have no effect on other chemicals that the body releases when exposed to allergens. For these reasons, antihistamines can be expected to reduce allergy symptoms by only about 50%.

In some people antihistamines become less effective when used over a long time. Switching to another type of antihistamine may help.

The antihistamines Seldane and Seldane-D were taken off the market in 1997 due to concerns that the active ingredient, terfenadine, triggered life-threatening heart rhythm problems when taken with certain drugs. Terfenadine can also be dangerous to people with liver disease. The U.S. Food and Drug Administration encouraged people who were using Seldane to talk to their physicians about switching to another antihistamine such as loratadine (Claritin) or to fexofenadine (Allegra), which is similar to Seldane but has a safer active ingredient.

The antihistamine astemizole (Hismanal) may also cause life-threatening heart rhythm problems or severe allergic reactions when taken in higher-than-recommended doses or with certain drugs. Taking astemizole with food may reduce its absorption into the bloodstream. Astemizole (Hismanal) should not be combined with any of the following:

  • the antibiotics erythromycin (E-Mycin and other brands), clarithromycin (Biaxin), or troleandomycin (TAO)
  • the blood pressure medicine mibefradil (Posicor)
  • medicines used in treating HIV infection such as indinavir (Crixivan), ritonavir (Norvir), and nelfinavir (Viracept)
  • antidepressants such as fluoxetine (Prozac), sertraline (Zoloft), or paroxetine (Paxil)
  • asthma medicines such as zileuton (Zyflo)
  • the antifungal drugs ketoconazole (Nizoral) or itraconazole (Sporanox)
  • large doses of quinine

In addition, patients with liver disease should not take Hismanal.

People with asthma, emphysema, chronic bronchitis, or other breathing problems should not use antihistamines unless directed to do so by a physician.

Some antihistamines make people drowsy, dizzy, uncoordinated, or less alert. For this reason, anyone who takes these drugs should not drive, use machines or do anything else that might be dangerous until they have found out how the drugs affect them.

Antihistamines can interfere with the results of skin and blood tests. The antihistamine promethazine (Phenergan) can interfere with pregnancy tests and can raise blood sugar. Anyone who is taking antihistamines should notify the health care provider in charge before scheduling medical tests.

People with phenylketonuria should be aware that some antihistamine products contain aspartame (Nutrasweet), which breaks down in the body to phenylalanine.

Anyone who has sleep apnea (periods when breathing stops during sleep) should not take the antihistamine promethazine (Phenergan). This drug may also cause people with seizure disorders to have more frequent seizures.

Because children are often more sensitive to antihistamines, they may be more likely to have side effects and to suffer from accidental overdoses. Check with a physician before giving antihistamines to children under 12 years.

Older people may also be more likely to have side effects, such as nervousness, irritability, dizziness, sleepiness, and low blood pressure from antihistamines. Older men may also have problems urinating. Unless these problems are severe, they can usually be handled by taking a lower dose or switching to a different antihistamine. However, people over 80 or older people with serious physical problems or dementia may become confused, disoriented, and incoherent after taking even small amounts of the antihistamine diphenhydramine (Benadryl).

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Science EncyclopediaScience & Philosophy: Anticolonialism in Southeast Asia - Categories And Features Of Anticolonialism to Ascorbic acidAntihistamines - Recommended Dosage, Precautions, Side Effects, Interactions - Special conditions