Isoniazid (isonicotinylhydrazine or INH), is a medication administered as a first line of defense against tuberculosis, a disease which infects nearly one-third of the world's population, according to the U.S. Centers for Disease Control and Prevention. Isoniazid is a prodrug, meaning that, once in the body, it must be activated by an enzyme produced by Mycobacterium tuberculosis. Isoniazid is never administered as the sole treatment, rather several antimycobiotic medication are given together as treatment to prevent the bacteria from becoming resistant to the drugs. Isoniazid causes a variety of side effects, some of them severe, especially when used for extended periods of time.
Gastrointestinal
Isoniazid can disrupt the normal functions of the intestines causing nausea, vomiting and constipation. Isoniazid is available as tablets, syrup or in solution for injection. The syrup is known to induce diarrhea, but this effect is likely due to the sorbitol, a sugar alcohol, used in the preparation of the syrup, according to the American Thoracic Society, CDC, and Infectious Diseases Society of America.
Central Nervous System
Isoniazid is known to cause problems associated with the central nervous system. These side effects range from mild irritability, lack of concentration and dysphoria (a general feeling of discontent) to depression. Isoniazid may also cause dysarthria, a speech impairment caused by a weakening of the muscles used to articulate words. A more serious side effect involving the central nervous system is the onset of seizures.
Isoniazid may cause peripheral neuropathy, a condition in which nerves in the peripheral nervous system are damaged. This can result in a numbing, tingling or burning sensation in the fingers or toes. This side effect is more common in people with other diseases such as HIV, diabetes, renal failure or alcoholism.
These central nervous system side effects might be due to the fact that isoniazid interferes with the breakdown of pyridoxine (vitamin B6) in the body. Therefore, patients taking isoniazid are often given vitamin supplements to help combat these side effects.
Hepatitis
Isoniazid, especially when given in conjunction with rifampin (another first-line medication used to treat tuberculosis), may cause liver toxicity resulting in hepatitis (inflammation of the liver). Patients who have an underlying liver disease or who consume large amounts of alcohol are at a higher risk of this side effect, according to the American Thoracic Society, CDC, and Infectious Diseases Society of America. The risk of hepatitis also increases with age with 2 percent of patients between the ages of 50 and 64 experiencing hepatitis due to taking isoniazid alone.
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