If a doctor just prescribed Isoniazid, you probably want two things: confidence it works and a clear plan to take it safely. This medicine is a backbone drug against tuberculosis (TB), but it’s also the one people worry about because of its liver warnings. The good news: with smart monitoring, vitamin B6, and a few simple rules, most people finish treatment without trouble. I’ll show you how to do that-step by step, with examples, checklists, and what to do if things go off script. I live in Hobart, so I’ll point out a couple of Australian specifics as we go.

TL;DR - What you need to know fast

• What it is: A first-line TB antibiotic used for active disease (with other drugs) and latent TB infection (LTBI) on its own or in short rifamycin-based combos.
• How to take: Usually 300 mg once daily for adults (10-15 mg/kg in kids). Take on an empty stomach if you can. Add vitamin B6 (pyridoxine) if you’re at risk for nerve symptoms.
• Risks to watch: Liver inflammation and peripheral neuropathy. Stop and call your clinician if you get dark urine, yellow eyes/skin, right-upper belly pain, severe nausea/vomiting, or tingling that spreads.
• Monitoring: Monthly symptom checks. Do baseline and follow-up bloods if you have risk factors (age >35-50, alcohol use, pregnancy/postpartum, liver disease, HIV, diabetes, or on interacting meds).
• Alternatives: For LTBI, shorter regimens like 3 months of once-weekly isoniazid + rifapentine (3HP) or 3 months of daily isoniazid + rifampicin (3HR) are often easier to complete.

How to take isoniazid safely (step-by-step)

Here’s a simple path you can follow with your clinician. It works for both latent TB infection and active TB (noting that active TB always uses a combination of drugs).

1. Confirm the job. Are you treating latent infection or active disease? Active TB needs a multi-drug plan (usually isoniazid + rifampicin + pyrazinamide + ethambutol to start). Latent TB may be isoniazid alone for 6-9 months or a shorter combo with a rifamycin.
2. Baseline checks. Share your history: alcohol intake, pregnancy/postpartum status, liver disease, HIV, diabetes, past hepatitis, and all medicines/supplements. If you have risk factors, get baseline liver tests (ALT/AST, bilirubin). Many services also check baseline in anyone over 35-50.
3. Add vitamin B6 (pyridoxine) when needed. Adults at risk for nerve symptoms (pregnancy, diabetes, HIV, kidney disease, malnutrition, alcohol use, older age, or on isoniazid for LTBI) typically take 25-50 mg pyridoxine daily. Kids usually take 1 mg/kg/day (max 25 mg).
4. Pick the dose. Adults: 300 mg once daily (or 5 mg/kg, max 300 mg). Children: 10-15 mg/kg once daily (max 300 mg). For active TB, your clinic may use thrice-weekly dosing under supervision; follow their schedule exactly.
5. Timing and food. Take on an empty stomach (1 hour before or 2 hours after food) for best absorption. If this upsets your stomach, take it with a light snack-better to take it than miss it.
6. Alcohol: keep it minimal or avoid. Alcohol raises the risk of liver injury. If you drink, keep it to very small amounts and discuss with your clinician. Many programs recommend avoiding alcohol during therapy.
7. Watch for symptoms each week. Liver: fatigue, nausea, poor appetite, dark urine, pale stools, yellow eyes/skin, right-sided upper belly pain. Nerves: tingling, burning, numbness in hands/feet. Eyes: blurred vision (rare; think ethambutol more than isoniazid, but call if it happens). Skin: rash or fever. Mood changes or confusion-rare but urgent.
8. When to stop immediately and call. If you develop hepatitis symptoms, severe abdominal pain, persistent vomiting, jaundice, or rapidly worsening neuropathy, stop the tablets and contact your clinician or go to urgent care.
9. Blood test triggers. If you have symptoms and ALT/AST are 3 times the upper limit of normal (ULN) or more, most guidelines say stop. If you have no symptoms and ALT/AST are 5× ULN or more, stop and reassess. Your clinician will confirm and guide next steps.
10. Missed doses. Daily dosing: if you remember within several hours, take it; if it’s close to the next dose, skip and take the next dose at the usual time. Don’t double up. For weekly regimens like 3HP, call your clinic if you miss a dose-schedules are strict.
11. Finish the course. For LTBI, commit to the full plan (e.g., 6 or 9 months daily, or 12 weekly doses with 3HP, or 3 months daily with 3HR). For active TB, stick with the combination plan and the continuation phase-your team will set the timeline.
12. Document your course. Keep a simple log of doses taken, any side effects, and clinic visits. This helps your team make fast, safe calls if anything changes.

Doses, regimens, and real-world scenarios

Standard dosing

Adults typically take 300 mg once daily, which equals roughly 5 mg/kg up to a 300 mg max. Children take 10-15 mg/kg daily (max 300 mg). For active TB, once the intensive phase finishes, your team may keep isoniazid with rifampicin for the continuation phase.

Latent TB options you’ll hear about

• 6H or 9H: Isoniazid daily for 6 or 9 months. 9H has a slightly higher protection rate but more dropouts. 6H is often used if adherence looks strong and risk is moderate.
• 3HR: Isoniazid + rifampicin daily for 3 months. Shorter, with high completion rates. Check interactions with rifampicin (it can reduce the effect of many meds, including some contraceptives and warfarin).
• 3HP: Isoniazid + rifapentine once weekly for 12 doses. Often supervised or self-administered with follow-up. Popular because it’s short.
• 4R: Rifampicin alone daily for 4 months. A non-isoniazid option if isoniazid isn’t suitable.

Which one? For many adults with LTBI in 2025, 3HR or 3HP is preferred because it’s shorter and people are more likely to finish. If rifamycins clash with your other medicines, classic isoniazid-only regimens still work well when taken to completion. National and WHO guidance back these choices (WHO 2023 consolidated TB guidelines; CDC 2024 LTBI recommendations; Australian Therapeutic Guidelines 2024).

Special groups and how I think about them

• Pregnancy: Isoniazid is generally considered safe. Add pyridoxine, and monitor liver enzymes, especially in the late third trimester and the first 3 months postpartum when hepatitis risk is higher. Treat LTBI during pregnancy if the risk of progression to active TB is high; otherwise, some clinicians defer until postpartum.
• Breastfeeding: Compatible. Drug passes into milk in small amounts. Mothers on isoniazid usually take pyridoxine; breastfed infants on isoniazid prophylaxis get pyridoxine too.
• HIV: Treat TB as a priority; drug interactions are common with antiretrovirals (less so with isoniazid than with rifamycins). B6 is a must. Link care across TB and HIV teams.
• Diabetes, kidney disease, malnutrition, older age: Higher risk of neuropathy and liver issues. Use B6 and do closer monitoring.
• Pre-existing liver disease or regular alcohol use: Discuss risk vs benefit. Many still complete therapy safely with tight monitoring or a rifampicin-based alternative if suitable.

Common interactions that change the plan

Isoniazid can raise blood levels of phenytoin and carbamazepine (seizure meds), and can increase the effect of warfarin. Disulfiram plus isoniazid has been linked to psychosis-avoid this combo. Alcohol raises liver risk. Paracetamol (acetaminophen) is okay at normal doses but use sparingly while on therapy. Rifampicin (if you’re taking it) lowers the effect of many drugs (e.g., some contraceptives, warfarin, certain antiretrovirals); your clinician will adjust.

Real-world scenarios

• Healthy 25-year-old with LTBI: 3HP or 3HR often wins due to short duration. If choosing 6H or 9H, add B6 if risk factors exist; monthly symptom checks suffice.
• 68-year-old with diabetes who enjoys wine: Consider 3HR or 4R to shorten exposure. If using isoniazid, add B6, minimize alcohol, check baseline LFTs, and recheck at 2-4 weeks then monthly.
• Pregnant woman in second trimester with recent TB exposure: If high risk, start isoniazid now with B6 and monitor closely; otherwise, discuss deferring until 2-3 months postpartum when safe to do so. Active TB is treated during pregnancy without delay.
• Man on phenytoin for seizures: If isoniazid is needed, check phenytoin levels and symptoms (nystagmus, ataxia, slurred speech). Dose adjustments may be needed.
• Asymptomatic ALT rise to 3× ULN at week 4: Pause therapy. Recheck labs, review alcohol/meds, discuss alternatives or re-challenge when safe per your specialist’s advice.

Evidence anchors

Guidance here aligns with WHO 2023 TB guidelines, CDC 2024 LTBI treatment recommendations, the ATS/CDC/IDSA TB guidance, Australia’s Therapeutic Guidelines (Antibiotic/TB 2024), and the NIH LiverTox review (updated through 2024). Rates of clinically significant isoniazid hepatitis sit around 0.1-1%, rising with age, alcohol, and postpartum status; mortality is rare with modern monitoring.

Quick checklists, numbers, and pro tips

Start-of-therapy checklist

• Confirm indication: LTBI vs active TB (active TB = combination therapy).
• List all meds, alcohol use, and supplements; flag phenytoin, carbamazepine, warfarin, disulfiram.
• Baseline labs if risk factors: ALT/AST, bilirubin; pregnancy test if relevant.
• Plan for B6: Adults 25-50 mg daily if at risk; kids 1 mg/kg (max 25 mg).
• Choose regimen: 6H/9H, 3HR, 3HP, or alternatives, based on interactions and your ability to complete the course.
• Set monitoring schedule: monthly symptom checks; earlier review if any red flags.

Monthly review checklist

• Any signs of liver trouble: nausea, low appetite, dark urine, pale stools, yellowing, right-upper belly pain?
• Any tingling, burning, numbness in hands/feet?
• Any new medicines, supplements, or alcohol changes?
• Adherence: how many doses missed? Fix the routine if needed.
• For those with risk factors: repeat liver tests as planned.

Stop-or-hold rules (talk to your clinician)

• Symptoms + ALT/AST ≥3× ULN → hold and reassess.
• No symptoms + ALT/AST ≥5× ULN → hold and reassess.
• Rapidly worsening neuropathy → hold; increase B6; consider alternative regimen.
• Severe rash, fever, or signs of hypersensitivity → stop and seek care.

Pro tips that save headaches

• Set a daily alarm and pair your dose with a fixed routine (e.g., teeth brushing).
• Keep alcohol to none or tiny amounts-liver risk drops fast when you do.
• If morning dosing makes you nauseous, switch to evening on an emptier stomach, or try with a light snack.
• B6 isn’t just a box to tick-use it when you have risk factors, and it often prevents tingling symptoms.
• On rifampicin too? Back-up contraception is smart; rifampicin can reduce hormonal contraceptive effectiveness.
• Travelling? Carry a letter listing your TB meds; airport staff are used to seeing these.

Key numbers at a glance

Examples that map to your situation

You work full-time and forget pills: A 3-month combo (3HR) or weekly 3HP may be easier than 9 months of daily tablets. Set alarms and use a pill box. For 3HP, missing a weekly dose matters-your clinic will reschedule so you still hit 12 doses.

You drink socially on weekends: Aim for none while you’re on treatment. If you choose to drink, keep it very light and infrequent. Arrange liver tests if you notice any nausea or fatigue that lingers.

Your baseline ALT is slightly elevated: You can often proceed with isoniazid if benefits outweigh risks, but schedule early repeat labs (e.g., 2-4 weeks) and avoid alcohol and unnecessary paracetamol. Your clinician may choose a rifampicin-only regimen if interactions allow.

You’re 3 weeks postpartum: You can take isoniazid with B6 while breastfeeding. Because postpartum hepatitis risk is higher, report any appetite loss or nausea immediately and consider earlier lab checks.

You’re on warfarin: Isoniazid may increase INR; rifampicin (if co-prescribed) lowers INR. You may need more frequent INR checks and dose tweaks.

Your fingers started tingling after 2 weeks: Tell your clinician. They may increase B6, check your blood sugar, and consider a brief hold if symptoms escalate. Most tingling improves with B6 and time.

FAQ and what to do next

Can I take it with food? Yes, if an empty stomach makes you queasy. Absorption is a bit better without food, but taking it with a light snack is fine if it helps you stay on track.

Can I drink alcohol? Best to avoid. Alcohol increases liver risk. If you do drink, keep it minimal and talk to your clinician.

What about paracetamol? Okay in normal doses, but don’t overuse it while on therapy. If you’re taking it daily, mention it to your clinician so they can plan monitoring.

Is it safe in pregnancy? Yes, with B6 and monitoring. The postpartum period (first 3 months) brings higher liver risk-watch for symptoms and keep your appointments.

Does it affect birth control? Isoniazid itself doesn’t reduce the pill’s effectiveness. Rifampicin (if you’re on it) does, so use condoms or a non-hormonal backup while on rifampicin and for some time after.

Foods to avoid? Some people flush or feel odd after foods rich in histamine or tyramine (aged cheeses, cured meats, certain fish, red wine) due to mild MAO inhibition. It’s not common; if it happens, limit those foods.

Can I split the dose? Stick to once daily unless your clinician says otherwise. For supervised regimens, follow their schedule exactly.

How fast does liver injury resolve if I stop? Symptoms usually improve within days to weeks; lab values trail behind. Your clinician will guide when or if to re-challenge or switch drugs.

Do I need blood tests? If you’re young, healthy, and not drinking, many clinics just do monthly symptom checks. If you’re older than about 35-50, pregnant/postpartum, drink alcohol, or have liver risks or interacting meds, you’ll likely get baseline and follow-up labs.

Is it on the PBS in Australia? Yes, TB medicines are subsidised. Your TB clinic or GP can explain the script and costs.

Troubleshooting: if things don’t go to plan

Scenario: ALT/AST rise without symptoms (2-4× ULN)

Pause and reassess if ≥5× ULN without symptoms. If it’s under that and you’re feeling well, many clinicians repeat labs in a few days, review alcohol and paracetamol, and decide together whether to keep going. Never ignore new nausea, fatigue, or jaundice-symptoms change the rules.

Scenario: You develop tingling in your feet

Tell your clinician promptly. They may increase B6 (for adults, often to 50 mg/day), check vitamin levels, glucose, and thyroid, and ensure you’re not on other neuropathy triggers. If symptoms escalate or affect function, a short hold or switch may be needed.

Scenario: You missed several doses

Don’t double up. For daily LTBI regimens, pick up from today and extend the end date to hit the total planned doses. For weekly 3HP, the program will reschedule missed doses-call them.

Scenario: You need surgery or new meds

Bring your medication list to every appointment. Surgeons and anaesthetists want to know about TB drugs. If you’re starting phenytoin, carbamazepine, or warfarin, plan extra monitoring.

Scenario: Rash or fever

Mild rash: pause and call your clinician. Severe rash, facial swelling, fever, mucosal involvement, or breathing issues: seek urgent care.

When to seek urgent help

• Yellowing of eyes/skin, dark urine, pale stools
• Severe or persistent nausea, vomiting, or belly pain
• Confusion, severe headache, new vision changes
• Rapidly worsening numbness or weakness
• Severe rash or swelling

Where this guidance comes from

This playbook reflects current guidance from WHO’s 2023 TB recommendations, CDC’s 2024 LTBI updates, ATS/CDC/IDSA TB guidelines used by respiratory and infectious disease teams, Australia’s Therapeutic Guidelines (2024), and the NIH LiverTox review (updated through 2024). These sources shape the dosing, monitoring, and stop/restart thresholds clinicians use in practice.

You can get through this. Pick the right regimen, take B6 if you’re in a risk group, keep alcohol out of the picture, and check in monthly. That’s how people finish treatment, protect their liver, and move on.