isoniazid, drug used to treat tuberculosis. Also known as isonicotinic acid hydrazide, isoniazid is the most effective antituberculosis drug currently available. The drug inhibits or kills the tubercle bacilli that cause the disease. It is usually given together with some other antituberculosis drug such as streptomycin or aminosalicylic acid to prevent emergence of drug resistant organisms (see drug resistance). To prevent development of tuberculosis in individuals who have a positive reaction to a tuberculin skin test, isoniazid is given alone. Side effects are seen only with very high doses.