Isoniazid

Isoniazid, commonly abbreviated as INH, is one of the most important and widely used anti-tuberculosis medicines in the world. Discovered in the 1950s, it remains a cornerstone of TB treatment and prevention more than seven decades later. Isoniazid works by inhibiting the synthesis of mycolic acid, a critical component of the cell wall of Mycobacterium tuberculosis. Without an intact cell wall, the TB bacterium cannot survive. Isoniazid is bactericidal against actively growing TB bacteria (meaning it kills them) and bacteriostatic against dormant bacteria (meaning it prevents them from multiplying). This dual action makes it essential for both the treatment of active TB disease and the prevention of TB in people who carry latent TB infection. In Nigeria, where tuberculosis remains a leading cause of death — particularly among people living with HIV — isoniazid is a life-saving medicine that is freely available through the National Tuberculosis and Leprosy Control Programme (NTBLCP) at DOTS centres across all 36 states and the Federal Capital Territory.

In the treatment of active TB, isoniazid is never used alone — it is always part of a multi-drug combination to prevent the development of drug resistance. The standard first-line regimen for drug-susceptible TB in Nigeria consists of two months of intensive treatment with four drugs — Rifampicin, Isoniazid, Pyrazinamide, and Ethambutol (known as the RHZE combination) — followed by four months of continuation phase treatment with Rifampicin and Isoniazid (RH). Fixed-dose combination tablets containing isoniazid with other anti-TB drugs are commonly used in Nigeria to simplify dosing and improve adherence. Beyond treating active TB, isoniazid has a uniquely important role in TB prevention through Isoniazid Preventive Therapy (IPT). In Nigeria, IPT is a critical intervention for people living with HIV, as TB is the leading cause of death among HIV-positive individuals. The Nigerian national guidelines recommend that all HIV-positive patients who have been screened and found not to have active TB should receive IPT — typically isoniazid 300mg daily for 6 months — to prevent the development of active TB disease.

A key consideration with isoniazid use is its potential to cause peripheral neuropathy — a painful condition characterised by numbness, tingling, and burning sensations in the hands and feet. This side effect occurs because isoniazid interferes with the metabolism of Vitamin B6 (pyridoxine), which is essential for nerve function. To prevent this complication, isoniazid is always co-prescribed with pyridoxine (Vitamin B6) supplements, typically 25mg to 50mg daily for adults. This is standard practice at DOTS centres and ART clinics throughout Nigeria, from teaching hospitals in Ibadan, Jos, and Maiduguri to community health centres in rural Benue, Taraba, and Cross River states. Patients must be counselled to take their pyridoxine supplement consistently alongside their isoniazid. Another important concern is isoniazid-related hepatotoxicity (liver damage), which can range from a mild, self-limiting elevation of liver enzymes to fulminant hepatic failure. Liver function monitoring is recommended, particularly during the first three months of treatment, in patients over 35 years of age, those with pre-existing liver disease, heavy alcohol users, and patients co-infected with HIV and Hepatitis B or C.