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Antihistamines may increase the effects of other drugs that slow down the central nervous system (CNS), such as alcohol, tranquilizers, barbiturates, and sleep aids. Avoid drinking alcohol while taking antihistamines, and check with a physician before combining antihistamines with other CNS depressants.

Certain antihistamines should not be used within two weeks of using monoamine oxidase inhibitors (MAO inhibitors), which are drugs used to treat Parkinson's disease, depression, and other psychiatric conditions. Examples of MAO inhibitors are Parnate and Nardil. People who have been taking MAO inhibitors or who are not sure if they have should check with their physician or pharmacist before taking antihistamines.

Although no such interactions have been reported, antihistamine loratadine (Claritin) could potentially interact with the antiulcer drugs cimetidine (Tagamet) and ranitidine (Zantac), both of which are also taken for heartburn; with antibiotics such as erythromycin and Biaxin; with the antifungal drug ketoconazole (Nizoral); and with the bronchodilator theophylline (Theo-Dur). These interactions can cause liver problems. Persons who are taking these drugs should consult with an physician or pharmacist to see whether this is an issue for them.

Check with a physician before combining the antihistamine chlorpheniramine (Deconamine) with any of the following:

  • asthma medicines such as albuterol (Proventil) and bromocriptine (Parlodel)
  • blood pressure drugs such as mecamylamine (Inversine), methyldopa (Aldomet), and reserpine
  • narcotic painkillers such as meperidine (Demerol), oxycodone-aspirin (Percodan) and oxycodone-acetaminophen (Percocet)
  • the anticonvulsant drug phenytoin (Dilantin)
  • sleep aids such as triazolam (Halcion) and secobarbital (Seconal)
  • tranquilizers such as diazepam (Valium) and alprazolam (Xanax)

Hismanal should not be taken with grapefruit juice or combined with any of the following drugs:

  • antibiotics such as 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

Check with a physician before combining any antihistamine with any of the following drugs:

  • Medicines for stomach or abdominal cramps or spasms, such as dicyclomine (Bentyl).
  • The antibiotics azithromycin (Zithromax), clarithromycin (Biaxin), or erythromycin (E-mycin).
  • The antifungal drugs itraconazole (Sporanox) or ketoconazole (Nizoral).
  • The calcium channel blocker bepridil (Vascor).
  • Drugs used to treat irregular heartbeat, such as disopyramide (Norpace), procainamide (Pronestyl), or quinidine (Quinaglute Dura-Tabs, Cardioquin).
  • Antidepressants such as maprotiline (Ludiomil) or tricyclic antidepressants such as desipramine (Norpramin) or imipramine (Tofranil).
  • Medicines called phenothiazines, used to treat mental, emotional, and nervous disorders. Examples are chlorpromazine (Thorazine) and prochlorperazine (Compazine).
  • Pimozide (Orap), used to treat symptoms of Tourette's syndrome.

Not all possible interactions of antihistamines with other drugs are listed here. Be sure to check with a physician or pharmacist before combining antihistamines with any other prescription or nonprescription (over-the-counter) medicine.

Nancy Ross-Flanigan


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—An otherwise harmless substance that can cause a hypersensitive allergic response.


—A sudden, life-threatening allergic reaction.


—A sensory experience of something that does not exist outside the mind. A person can experience a hallucination in any of the five senses. Hallucinations usually result from drugs or mental disorders.


—A chemical released from cells in the immune system as part of an allergic reaction.


—(PKU) A genetic disorder in which the body lacks an important enzyme. If untreated, the disorder can lead to brain damage and mental retardation.


—Dust-like grains produced by the male parts of plants and carried by wind, insects, or other methods, to the female parts of plants.

Additional topics

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