TB Drug Toxicity Monitoring Guide
Drug-susceptible active TB is treated with the four-drug RIPE regimen (rifampin, isoniazid, pyrazinamide, ethambutol) for a 2-month intensive phase, followed by rifampin plus isoniazid for a 4-month continuation phase (6 months minimum). Each first-line agent carries a signature toxicity — with hepatotoxicity shared among three of the four — so exam questions hinge on matching the correct adverse effect, its mechanism, the appropriate monitoring, and the specific preventive step to the right drug.
Isoniazid — hepatotoxicity & peripheral neuropathy
Isoniazid's major toxicities are hepatotoxicity (idiosyncratic liver injury) and peripheral neuropathy from pyridoxine (vitamin B6) depletion. Obtain baseline LFTs and repeat liver testing only in patients with risk factors (age >35, alcohol use, pre-existing liver disease, pregnancy/postpartum) or those who develop symptoms — routine monthly LFTs are not required for everyone. Educate all patients to report symptoms of hepatitis (nausea, dark urine, RUQ pain, jaundice). The classic preventive measure is co-administration of pyridoxine (B6) 25–50 mg daily to prevent the neuropathy. Isoniazid is also a mainstay of latent TB therapy (historically 9 months alone, or combined with rifapentine for a 3-month, 12-dose weekly regimen).
Rifampin — hepatotoxicity & drug interactions
Rifampin causes hepatotoxicity via cholestatic injury and, critically, is a potent CYP450 inducer, driving numerous drug interactions. Obtain baseline LFTs, repeat testing when risk factors or symptoms are present, and review interactions with all of the patient's other medications. A frequently tested board point: the orange/red discoloration of body fluids (urine, tears, sweat) is benign and does not require stopping the drug. Rifampin continues into the continuation phase alongside isoniazid and is also an option for latent TB (4 months, with fewer side effects than isoniazid).
Pyrazinamide — hepatotoxicity & hyperuricemia
Pyrazinamide contributes to hepatotoxicity and causes hyperuricemia by inhibiting renal urate excretion. Obtain baseline LFTs with repeat testing guided by risk factors or symptoms, and monitor uric acid. It should be stopped if severe hepatitis or symptomatic gout develops. Pyrazinamide is used only during the 2-month intensive phase, not the continuation phase.
Ethambutol — optic neuritis
Ethambutol's signature toxicity is optic neuritis, resulting from retinal ganglion cell toxicity, which manifests as decreased visual acuity and impaired color vision. Monitoring requires baseline and monthly assessment of visual acuity and color vision. Because reliable reporting of visual symptoms is essential to catch toxicity early, avoid ethambutol in children who cannot report symptoms.
The RIPE regimen framework
Standard drug-susceptible TB therapy is a 6-month minimum course: intensive phase with all four RIPE drugs for 2 months, then rifampin + isoniazid for a 4-month continuation phase. MDR-TB (resistant to both isoniazid and rifampin) requires prolonged treatment with second-line agents. Under the WHO's 2021 definition, XDR-TB is MDR/rifampin-resistant TB with additional resistance to any fluoroquinolone AND at least one additional Group A drug (bedaquiline or linezolid), and is very difficult to treat.
High-yield
- RIPE = Rifampin, Isoniazid, Pyrazinamide, Ethambutol — the standard intensive-phase regimen.
- Three of the four first-line TB drugs (isoniazid, rifampin, pyrazinamide) are hepatotoxic — obtain baseline LFTs; repeat only for risk factors or symptoms.
- Isoniazid → peripheral neuropathy from pyridoxine (B6) depletion; give B6 25–50 mg daily.
- Rifampin → CYP450 inducer causing drug interactions; orange discoloration of fluids is benign.
- Pyrazinamide → hyperuricemia by inhibiting urate excretion; monitor uric acid.
- Ethambutol → optic neuritis (retinal ganglion cell toxicity); baseline + monthly visual acuity and color vision.
- MDR-TB = resistant to isoniazid AND rifampin; XDR-TB (WHO 2021) adds fluoroquinolone resistance plus resistance to a Group A drug (bedaquiline or linezolid).
Pitfalls
- Confusing which drug causes optic neuritis — it's ethambutol, not isoniazid.
- Stopping rifampin because of orange body fluids — this discoloration is benign and expected.
- Forgetting pyridoxine with isoniazid, leading to preventable peripheral neuropathy.
- Overlooking rifampin's CYP450 induction and its interactions with a patient's other medications.
- Giving ethambutol to young children who cannot reliably report visual changes, delaying detection of optic neuritis.
- Attributing hyperuricemia/gout to the wrong agent — pyrazinamide is the drug that inhibits urate excretion.
- Assuming routine monthly LFTs are required for all patients — baseline testing is standard, with repeat testing reserved for those with risk factors or symptoms.
Clinical pearls
- Any new visual acuity or color vision change on TB therapy points to ethambutol.
- Peripheral neuropathy on TB therapy = isoniazid; the fix is pyridoxine.
- Check for drug interactions whenever a patient is started on rifampin.
- Rising uric acid on the RIPE regimen implicates pyrazinamide.
- LFTs are the shared monitoring thread for isoniazid, rifampin, and pyrazinamide — get a baseline, then follow clinically.
Frequently asked
Which TB drug requires pyridoxine supplementation and why?
Isoniazid — it depletes pyridoxine (vitamin B6), causing peripheral neuropathy. Give pyridoxine 25–50 mg daily to prevent it.
A patient on TB therapy notices orange urine and tears — what should you do?
Reassure the patient. Orange discoloration of body fluids is a benign effect of rifampin and does not require stopping the drug.
Which TB drug causes optic neuritis, and how is it monitored?
Ethambutol, via retinal ganglion cell toxicity. Monitor baseline and monthly visual acuity and color vision, and avoid it in children who cannot report visual symptoms.
Which first-line TB drugs are hepatotoxic, and how should the liver be monitored?
Isoniazid, rifampin, and pyrazinamide are all hepatotoxic. Obtain baseline LFTs; routine monthly LFTs are not required for everyone — repeat testing is reserved for patients with risk factors (age >35, alcohol use, pre-existing liver disease, pregnancy/postpartum) or those who develop symptoms. Stop the offending drug for severe hepatitis.
Why does rifampin cause so many drug interactions?
Rifampin is a potent CYP450 inducer, which accelerates metabolism of many other drugs. Always review interactions with all of the patient's medications.
Which drug causes hyperuricemia and what is the mechanism?
Pyrazinamide inhibits renal urate excretion, causing hyperuricemia. Monitor uric acid and stop the drug if severe symptomatic gout develops.
What is the standard regimen for drug-susceptible active TB?
A 2-month intensive phase of RIPE (rifampin, isoniazid, pyrazinamide, ethambutol) followed by a 4-month continuation phase of rifampin plus isoniazid — 6 months minimum.
How are MDR-TB and XDR-TB defined?
MDR-TB is resistant to both isoniazid and rifampin and requires prolonged second-line therapy. Under the WHO's 2021 definition, XDR-TB is MDR/rifampin-resistant TB with added resistance to any fluoroquinolone AND at least one additional Group A drug (bedaquiline or linezolid).
Turn this into reasoning you can use on exam day — practice TB Drug Toxicity Monitoring Guide on branching cases where your decisions shape the patient.