TB Drug Comparison: First‑Line, Second‑Line and Key Differences

When you or someone you know is diagnosed with tuberculosis, the first thing you hear is “you’ll need a drug combo.” That combo isn’t random – it’s a carefully balanced mix of medicines that hit the bug from different angles. Knowing how each drug works, its dose, and its side‑effects can make the whole treatment less scary.

First‑line TB drugs – the backbone of therapy

The standard 6‑month regimen uses four first‑line drugs: isoniazid (INH), rifampin (RIF), pyrazinamide (PZA) and ethambutol (EMB). Isoniazid is the star for killing actively growing bacteria, but it can cause liver irritation in about 10% of people. Rifampin boosts the kill‑rate and also hits hidden bacteria; it turns urine orange, which can surprise patients. Pyrazinamide works best in the acidic environment of TB lesions, but it’s the biggest culprit for joint pain and higher liver enzymes. Ethambutol protects the other drugs from resistance, but it may affect vision – a quick eye check at the start and during treatment catches problems early.

Typical adult doses are 5 mg/kg for isoniazid, 10 mg/kg for rifampin, 15–30 mg/kg for pyrazinamide, and 15 mg/kg for ethambutol, taken once daily. The first two months you take all four drugs (the intensive phase), then drop pyrazinamide and ethambutol for the next four months (the continuation phase). Sticking to the schedule is crucial; missing doses can let the bacteria become resistant.

Second‑line drugs – used when resistance shows up

If the bug resists any first‑line drug, doctors move to second‑line options. These include fluoroquinolones like levofloxacin or moxifloxacin, injectable agents such as amikacin, and oral drugs like cycloserine, ethionamide, and linezolid. Fluoroquinolones are often the go‑to because they’re easier to give by mouth and have good activity, but they can cause tendon pain and affect blood sugar. Injectable agents need a daily shot and can hurt the ears or kidneys, so they’re used only when necessary.

Dosing for second‑line meds varies widely. For example, levofloxacin is usually 750 mg once daily, while amikacin is 15 mg/kg daily by injection. Because these drugs have more serious side‑effects, doctors monitor blood tests, hearing, and kidney function closely. Treatment length can stretch to 18‑24 months, and patients need strong support to stay on track.

Choosing a regimen isn’t just about the bacteria; it’s also about the patient. Age, liver health, HIV status, and pregnancy all affect which drugs are safest. For someone with mild liver disease, a doctor might swap pyrazinamide for a different agent to protect the liver. Pregnant patients avoid streptomycin and sometimes ethambutol, opting for regimens that have proven safety.

Bottom line: understanding the main differences between TB drugs helps you ask the right questions and spot warning signs early. Always let your healthcare team know if you notice orange urine, vision changes, joint pain, or any new symptoms. With the right info and support, completing the full course becomes a lot more doable.