An antibiotic pill for tuberculosis (TB). It is combined with other TB drugs to treat active disease and is also used to prevent TB in people at high risk. It can cause severe, sometimes fatal liver injury and nerve damage felt as tingling in the hands and feet.
Isoniazid stops TB bacteria from making mycolic acids, an essential part of their cell wall. At normal treatment levels it kills the germs outright instead of only slowing their growth. It works on TB germs that are actively multiplying, whether they sit inside the body's cells or outside them. The bacteria can turn resistant through changes in a few of their genes, and this happens quickly when it is the only TB drug used.
Taken by mouth, it reaches its highest blood level within 1 to 2 hours and spreads into all body fluids, including the fluid around the brain and spinal cord and the fluid around the lungs. The body breaks it down mainly by acetylation, a process whose speed is set by a person's genes, so slow processors keep higher levels and have more side effects. It also competes with vitamin B6, and high doses can leave adults short of that vitamin. Nerve damage, felt first as tingling in the feet and hands, is its most common toxic effect and happens more often in slow processors, which is why vitamin B6 is given with it to people prone to nerve problems.
Isoniazid treats tuberculosis (TB), a serious bacterial infection that mainly affects the lungs. It works against every form of TB, as long as the germs causing it are not resistant to it. It is also given to prevent TB in people at high risk, such as people with HIV, close contacts of someone newly found to have contagious TB, and people whose TB skin test has recently turned positive. The goal is to kill the TB germs, since stopping treatment too early makes the disease more likely to come back.
It is taken by mouth as a tablet, and preventive treatment is recommended for people in these risk groups at any age. Active TB is always treated with several TB medicines at the same time, because treating it with this one drug alone lets the germs become resistant. Many people take their doses while a health care worker watches, which helps them finish the long course. It is not used in people with acute liver disease, and active TB has to be ruled out before preventive treatment starts.
Tingling or numbness in the hands and feet from nerve damage is the most common problem, along with nausea, vomiting, and stomach upset. Mild rises in liver enzymes on blood tests are common in the first months and usually settle even while the drug continues. The label's boxed warning is about severe and sometimes fatal hepatitis, which is linked to age, daily alcohol use, and the months after childbirth, and it also warns about severe skin reactions and a brain problem that affects balance and coordination. Seizures, damage to the nerve of the eye, blood disorders, and a lupus-like illness are rarer serious effects.
This is the boxed warning, the warning the FDA label prints first.
Severe and sometimes fatal hepatitis can occur, even after many months of therapy. Risk is age related (about 23 per 1,000 at ages 50 to 64) and higher with daily alcohol use, chronic liver disease, injection drug use, and in women, particularly Black and Hispanic women and after delivery. Interview patients monthly; for those 35 and older, measure AST and ALT before starting and periodically. Strongly consider stopping if liver tests rise beyond 3 to 5 times the normal upper limit, stop promptly for symptoms of liver injury, and defer preventive therapy in acute liver disease.
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