Isoniazid Uses: What It Is, Why It Matters, and How It Can Be Repurposed

If you’ve heard of isoniazid but aren’t sure why it matters beyond TB, you’re in the right spot. This guide breaks down the classic role of the drug, the newer ways doctors are using it, and what you should watch for if you ever take it.

Main Clinical Uses

The original job of isoniazid is to kill Mycobacterium tuberculosis, the bug that causes active tuberculosis. It’s usually given daily for six months and works by blocking the bacteria’s ability to make its cell wall. Because it’s cheap and works well, it’s a staple in TB treatment kits worldwide.

When someone has been exposed to TB but isn’t sick yet, doctors often give isoniazid alone for nine months. This preventive therapy, called latent TB treatment, cuts the chance of developing active disease by up to 90 %. It’s especially common for people with weak immune systems, like those on steroids or living with HIV.

Beyond the lungs, isoniazid is also used for TB that shows up in the brain (TB meningitis) or bones (Pott’s disease). In these cases the drug is combined with other antibiotics to make sure every hidden pocket of bacteria is cleared.

Off‑Label and Emerging Uses

Researchers have spotted a few surprising roles for isoniazid. Some neurologists give a short course after exposure to TB meningitis to protect the brain, even though guidelines vary by country.

A handful of small studies suggest isoniazid might help with certain mood disorders. The theory is that by affecting brain chemistry linked to inflammation, it could lift depressive symptoms in people who also have TB. It’s still experimental, but it shows how a classic drug can spark new ideas.

In the lab, isoniazid is a favorite tool for scientists studying enzymes that need vitamin B6. By blocking those enzymes, researchers can map out metabolic pathways and develop new drugs. While this isn’t a patient‑level use, it highlights the molecule’s flexibility.

Safety matters, so here’s what to keep an eye on. The most common side effect is liver irritation; doctors usually check liver enzymes before and during treatment. Nerve problems, like tingling or numbness, can happen too—adding vitamin B6 (pyridoxine) to the regimen usually prevents that.

Typical adult dosing is 300 mg once a day, taken with food to reduce stomach upset. If you have liver disease, the dose may be lowered, and the treatment length could change. Always finish the full course, even if you start feeling better, to avoid resistance.

Bottom line: isoniazid is a workhorse for TB, but its off‑label adventures show it can do more. Knowing the main uses, the possible new ones, and the safety steps helps you stay informed whether you’re a patient, a caregiver, or just curious about drug repurposing.