What are the side effects of first-line tuberculosis (TB) medications?

Side Effects of First-Line TB Medications

All four first-line TB drugs—isoniazid (INH), rifampin (RIF), pyrazinamide (PZA), and ethambutol (EMB)—carry significant adverse effects that require systematic monitoring, with hepatotoxicity being the most serious concern across INH, RIF, and PZA, while EMB primarily threatens vision.

Isoniazid (INH)

Major Adverse Effects

• Hepatotoxicity: The most serious reaction, occurring at higher rates in patients with underlying liver disease, alcohol use, or when combined with rifampin 1
• Peripheral neuropathy: Caused by pyridoxine (vitamin B6) depletion 2
• CNS toxicity: Can cause seizures due to disruption of GABAergic neurotransmission 2

Risk Factors

• Advanced age
• Pre-existing liver disease
• Alcohol consumption
• Malnutrition
• NAT2 slow acetylator status 2

Monitoring & Management

• Pyridoxine supplementation prevents peripheral neuropathy 2
• Baseline and serial liver function tests when used with rifampin or in patients with liver disease 1

Rifampin (RIF)

Major Adverse Effects

• Hepatotoxicity: Can cause severe liver injury, particularly when combined with INH 1
• Drug interactions: Induces hepatic microsomal enzymes, accelerating clearance of numerous medications including oral contraceptives, anticoagulants, anticonvulsants, protease inhibitors, and NNRTIs 3
• Flu-like syndrome, hemolytic anemia, acute renal failure: Uncommon at standard 10 mg/kg daily dosing but more frequent with intermittent high-dose therapy 3
• Orange discoloration: Colors urine, tears, sweat, and body fluids orange; can permanently stain soft contact lenses 3

Critical Drug Interactions

• Contraindicated with most HIV protease inhibitors due to decreased protease inhibitor levels and increased rifampin toxicity risk 3
• Reduces efficacy of hormonal contraceptives—alternative contraception required 3

Pyrazinamide (PZA)

Major Adverse Effects

• Hepatotoxicity: Can cause severe and prolonged liver injury, though frequency is slightly lower than INH or RIF 1
• Gastrointestinal symptoms: Nausea and vomiting common, especially at higher doses 1
• Hyperuricemia: Expected effect, generally without adverse consequences 1
• Nongouty polyarthralgia: Occurs in up to 40% of patients, usually responds to NSAIDs without requiring drug discontinuation 1
• Acute gouty arthritis: Rare except in patients with pre-existing gout (contraindication to use) 1
• Photosensitive dermatitis 1

Monitoring

• Liver chemistry monitoring required when used with rifampin or in patients with underlying liver disease 1
• Serum uric acid not routinely recommended but may serve as compliance marker 1

Ethambutol (EMB)

Major Adverse Effects

• Retrobulbar (optic) neuritis: Manifests as decreased visual acuity or impaired red-green color discrimination affecting one or both eyes 1
  • Dose-related: minimal risk at 15 mg/kg daily 1
  • Risk increases dramatically at doses >30 mg/kg daily (18% incidence) 1
  • Higher risk in renal insufficiency 1
• Peripheral neuritis: Rare 1
• Cutaneous reactions: Requiring discontinuation in 0.2-0.7% of patients 1

Monitoring Requirements

• Baseline visual acuity testing (Snellen chart) and color discrimination (Ishihara tests) 1
• Monthly questioning about visual disturbances including blurred vision or scotomata 1
• Monthly visual testing for:
  • Doses >15-25 mg/kg
  • Treatment duration >2 months
  • Any patient with renal insufficiency 1
• Immediate permanent discontinuation if any signs of visual toxicity 1

Special Considerations

• Generally not recommended in children <5 years whose visual acuity cannot be monitored 1
• Can be used safely in patients with hepatic disease 1

Hepatotoxicity Management Across All Drugs

Discontinuation Thresholds

• ALT ≥3× upper limit of normal WITH hepatitis symptoms (nausea, vomiting, abdominal pain) 4
• ALT ≥5× upper limit of normal WITHOUT symptoms 4
• Stop all hepatotoxic drugs immediately and evaluate carefully 4

Common Pitfalls

• Failing to exclude other causes of abnormal liver function (viral hepatitis, biliary disease, alcohol, other hepatotoxic drugs, herbal supplements) before attributing to TB drugs 4
• Continuing therapy despite warning signs

Overall Adverse Event Rates

Recent evidence shows 67.8% of patients experience grade ≥2 adverse reactions to first-line TB drugs 5. The most common are:

• Fatigue (53.2%)
• Itching (42.7%)
• Anorexia (41.7%)

Only 9.5% require treatment regimen changes due to adverse reactions, with most patients completing treatment successfully despite experiencing adverse effects 5.

High-Risk Populations

Older Adults

Experience significantly more adverse reactions including anorexia, dyspepsia, rash, dizziness, anemia, and abnormal hepatic/renal function tests 5. Age >45 years increases risk of gastrointestinal reactions (OR 1.55) 6.

Other Risk Factors

• Female gender: Higher risk of skin reactions (OR 1.78) and nervous system reactions (OR 1.65) 6
• HIV infection: Independent predictor of adverse reactions across all systems 6
• Alcohol use: Independent predictor of adverse reactions 6
• Extrapulmonary TB: Increased risk of gastrointestinal reactions (OR 2.41) 6

Rifabutin (Alternative Rifamycin)

When rifampin cannot be used due to drug interactions:

• Better tolerated with less hepatic enzyme induction than rifampin 3
• Adverse effects: Rash, gastrointestinal intolerance, neutropenia, myalgias, dysgeusia 3
• Uveitis and skin pigmentation: When combined with CYP-3A4 inhibitors like clarithromycin 3
• Still has drug interactions but more manageable with dose adjustments 3

Practical Monitoring Algorithm

1. Baseline: Visual acuity, color discrimination, liver function tests, complete blood count
2. Monthly visits: Question about visual changes, gastrointestinal symptoms, neurologic symptoms
3. Serial testing: Liver function tests in high-risk patients or when symptoms develop
4. Immediate evaluation: Any unexplained nausea, vomiting, abdominal pain with liver function testing 4