Winthrop Isoniazid contains isoniazid 300 mg, a core tuberculosis medicine. It treats active TB as part of a combination, and is taken alone to prevent TB in people at high risk — including many people living with HIV. It is taken with vitamin B6 to protect the nerves.
What is Winthrop Isoniazid?
Winthrop Isoniazid contains isoniazid 300 mg. “Winthrop” is the manufacturer; the active ingredient and strength are what matter. It is a Schedule 4 prescription medicine, also written as INH.
Isoniazid blocks the production of mycolic acids, waxy fatty acids that form the outer wall of Mycobacterium tuberculosis. That wall is what makes TB so hardy and so difficult to treat — it resists drying, disinfectants and most ordinary antibiotics. No human cell makes mycolic acids, so isoniazid attacks the organism very specifically.
It is among the most effective TB medicines available, and it has been a backbone of treatment for decades.
Isoniazid is almost never used alone to treat active TB. Active disease involves an enormous number of bacteria, among which a few will already be naturally resistant to any single medicine. Treat with one agent and those few survive and multiply, producing a fully resistant infection. That is why active TB is treated with four medicines together — isoniazid, rifampicin, pyrazinamide and ethambutol — so that resistance to one is covered by the others.
The exception is preventive therapy, where the bacterial load is tiny and isoniazid alone is appropriate.
What is Winthrop Isoniazid commonly used for?
Treating active tuberculosis:
- Pulmonary TB, as part of the standard combination.
- TB elsewhere in the body — lymph nodes, bone, abdomen, TB meningitis.
Treatment runs for at least six months, in two stages: an intensive phase of four medicines, then a continuation phase of fewer.
Preventing tuberculosis (TB preventive therapy):
- In people living with HIV, where it substantially reduces the risk of developing active TB and of dying from it. This is a major part of South African HIV care.
- In household contacts of someone with TB, particularly young children.
- In people with latent TB infection who are at raised risk of it becoming active.
Latent TB is worth explaining, because it is the source of most confusion about this medicine. Someone can carry TB bacteria that their immune system has contained: they are not ill, not infectious, and have no symptoms, but the bacteria are alive and can become active later — especially if immunity weakens. Preventive therapy kills those dormant bacteria before that happens.
So if you have been given isoniazid and feel completely well, that is expected. You are not being treated for an illness you have; you are being prevented from developing one, and stopping early because you feel fine removes exactly the protection it exists to provide.
Finishing the full course is the whole point — for active TB, incomplete treatment is the principal driver of drug-resistant TB, which is far harder, longer and more toxic to treat.
How Winthrop Isoniazid fits into this medicine category
| Group | Examples | Role |
|---|---|---|
| Anti-TB medicines | Isoniazid, rifampicin, pyrazinamide, ethambutol | TB treatment and prevention |
| Broad-spectrum penicillins | Amoxicillin | Chest, ear, throat, dental |
| Macrolides | Azithromycin | Chest infections, penicillin allergy |
| Sulfonamide combinations | Co-trimoxazole (Purbac) | PCP prevention in HIV |
| Nitroimidazoles | Metronidazole | Anaerobic infections |
TB medicines are a distinct group — ordinary antibiotics do not treat TB. See the antibiotics and antimicrobials hub.
What to check before using Winthrop Isoniazid
Take your vitamin B6 (pyridoxine). Isoniazid interferes with vitamin B6, and the resulting deficiency causes peripheral neuropathy — numbness, tingling and burning in the hands and feet, which can become permanent. Pyridoxine is prescribed alongside to prevent this, and it is not optional. Risk is higher in people with HIV, diabetes, malnutrition, kidney disease, pregnancy, or who drink heavily.
Take it on an empty stomach — an hour before food or two hours after — as food reduces absorption.
Do not miss doses, and do not run out. Collect repeats before your supply ends.
Tell your prescriber or pharmacist if:
- You have liver disease, or drink alcohol regularly. Alcohol substantially increases the risk of liver injury.
- You have HIV, diabetes, kidney disease, or are malnourished.
- You have epilepsy or a history of seizures.
- You are pregnant or breastfeeding — TB treatment continues in pregnancy, but pyridoxine is essential.
- You take phenytoin, carbamazepine, warfarin, theophylline, or an antacid — space antacids by several hours.
- You take HIV medicines — tell the clinician managing both, since TB and HIV treatment interact.
- You are over 35, where the risk of liver effects rises.
Expect liver monitoring, particularly early in treatment and if you have risk factors.
Follow the dispensing label. This page does not set doses.
Possible cautions and when to ask a pharmacist or doctor
Liver injury is the main serious risk. Isoniazid can cause hepatitis, occasionally severe. Stop the medicine and contact your clinic immediately if you develop yellowing of the skin or eyes, dark urine, pale stools, persistent nausea or vomiting, loss of appetite, pain in the upper right abdomen, or unusual tiredness. Do not wait for a scheduled appointment.
Risk rises with age, alcohol, existing liver disease, and combination with rifampicin.
Nerve symptoms. Report numbness, tingling or burning in the hands or feet promptly — this responds to pyridoxine if caught early, and can persist if ignored.
Never stop TB treatment on your own. Interrupted or incomplete treatment is how drug-resistant TB develops, and resistant TB requires far longer treatment with more toxic medicines. If side effects are troubling you, or if cost, travel or work make collection difficult, tell the clinic — these are common, solvable problems, and TB services exist to help with exactly this.
Tell close contacts. With active TB, household members and close contacts should be screened. Young children and people with HIV are most at risk.
A rash, fever or joint pains should be reported.
Seek urgent medical attention for yellowing of the skin or eyes, dark urine with pale stools, persistent vomiting, severe abdominal pain, numbness or weakness in the hands or feet, a seizure, confusion, a widespread or blistering rash, or coughing up blood.
Related medicine guides
Frequently asked questions
What is Winthrop Isoniazid used for? Treating active tuberculosis as part of a combination, and preventing TB in people at high risk, including many people living with HIV.
Why am I taking it if I am not sick? That is TB preventive therapy. It kills dormant TB bacteria before they can become active disease. Feeling well is the intended result.
Why do I need vitamin B6 with it? Isoniazid interferes with vitamin B6, and the deficiency causes nerve damage in the hands and feet. Pyridoxine prevents that, and it is not optional.
Can I drink alcohol while taking it? No. Alcohol substantially increases the risk of liver injury, which is isoniazid’s most serious side effect.
Can I stop when I feel better? No. Incomplete TB treatment is the main cause of drug-resistant TB, which is far harder and longer to treat. Finish the full course.
What if I get side effects? Tell your clinic rather than stopping on your own. Yellowing of the skin or eyes, dark urine or persistent vomiting need immediate attention.