When a doctor needs to treat tuberculosis, the first drug that often comes to mind is Isoniazid. But the landscape of TB therapy includes several other agents, each with its own strengths and drawbacks. This guide walks you through the most common alternatives, compares their key attributes, and helps you decide which one fits a particular case.

What is Isoniazid?

Isoniazid is a bactericidal antibiotic specifically active against Mycobacterium tuberculosis. It is classified as a first‑line anti‑TB drug and is often used in both active disease and latent infection.

• Typical adult dose: 5mg/kg (max 300mg) daily.
• Key side effects: hepatotoxicity, peripheral neuropathy (preventable with pyridoxine).
• Mechanism: inhibits mycolic acid synthesis, compromising the bacterial cell wall.

How Rifampin

Rifampin is a broad‑spectrum antibiotic that also targets the bacterial RNA polymerase. It is a cornerstone of the standard four‑drug regimen for active TB.

• Typical adult dose: 10mg/kg (max 600mg) daily.
• Key side effects: orange‑red body fluids, liver enzyme elevation, drug‑drug interactions.
• Use: active TB, also effective for latent infection when combined with Isoniazid.

Understanding Ethambutol

Ethambutol interferes with the bacterial cell wall by blocking arabinogalactan synthesis. It is added to prevent resistance when the patient’s bacterial load is high.

• Typical adult dose: 15-25mg/kg daily.
• Key side effects: optic neuritis (vision changes), which are reversible if detected early.
• Role: part of the initial intensive phase in drug‑sensitive TB.

Pyrazinamide

Pyrazinamide works best in acidic environments, such as the intracellular compartments where TB bacteria hide. It shortens the overall treatment duration.

• Typical adult dose: 20-25mg/kg daily for the first two months.
• Key side effects: hyperuricemia, hepatotoxicity.
• Use: intensive‑phase drug in the standard regimen.

When Streptomycin

Streptomycin is an injectable aminoglycoside that disrupts protein synthesis. It is now reserved for multidrug‑resistant (MDR) TB or when oral options are limited.

• Typical adult dose: 15mg/kg intramuscularly daily.
• Key side effects: ototoxicity, nephrotoxicity.
• Current status: not part of first‑line therapy in most countries.

Exploring Levofloxacin

Levofloxacin is a fluoroquinolone with activity against TB strains resistant to first‑line drugs. It is taken orally and penetrates well into lung tissue.

• Typical adult dose: 750mg daily.
• Key side effects: tendon rupture, QT prolongation, gastrointestinal upset.
• Indication: MDR‑TB regimens, alternative when rifampin cannot be used.

The newer Bedaquiline

Bedaquiline targets the ATP synthase of Mycobacterium tuberculosis, a mechanism not shared by older drugs. It has become a vital component of the all‑oral MDR‑TB regimen.

• Typical adult dose: 400mg daily for 2 weeks, then 200mg three times per week.
• Key side effects: QT interval prolongation, hepatic enzyme elevation.
• Regulatory status: approved by WHO for MDR‑TB under careful cardiac monitoring.

The disease context: Tuberculosis

Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis, affecting primarily the lungs but capable of spreading to any organ. Treatment success hinges on selecting the right drug combination for the right duration.

Side‑by‑side comparison of the main agents

Choosing the right drug: practical decision points

Here are some concrete scenarios and what they imply for drug selection:

• Latent infection in a healthy adult: Isoniazid alone for 6‑9months (or a 3‑month weekly Isoniazid‑rifapentine combo). No need for the other agents.
• Newly diagnosed drug‑sensitive pulmonary TB: The classic four‑drug regimen (Isoniazid, Rifampin, Ethambutol, Pyrazinamide) for two months, followed by Isoniazid‑Rifampin for four months.
• Hepatic impairment: Reduce or omit Isoniazid and Pyrazinamide; consider a regimen based on Rifampin, Ethambutol, and a fluoroquinolone.
• Pregnancy: Isoniazid and Rifampin are generally safe; avoid Streptomycin because of fetal ototoxicity.
• MDR‑TB (resistance to Isoniazid and Rifampin): Build an all‑oral regimen including Bedaquiline, Levofloxacin, and possibly linezolid, guided by susceptibility testing.

Monitoring and managing side effects

Regardless of the chosen drug, regular follow‑up is essential.

1. Baseline liver function tests (ALT/AST) before starting Isoniazid, Rifampin, or Pyrazinamide.
2. Visual acuity and color vision testing at month 1 and month 2 when Ethambutol is part of the regimen.
3. Serum uric acid levels if Pyrazinamide is used for more than six weeks.
4. Quarterly ECGs for patients on Bedaquiline or Levofloxacin, especially if they have cardiac risk factors.
5. Peripheral neuropathy assessment in patients on Isoniazid; give pyridoxine 25mg daily to prevent nerve damage.

Common pitfalls and how to avoid them

Even seasoned clinicians can trip up. Below are typical mistakes and quick fixes.

• Skipping pyridoxine with Isoniazid: Leads to numbness or tingling. Always prescribe 25mg pyridoxine daily.
• Not checking drug interactions with Rifampin: Rifampin induces many cytochromeP450 enzymes, lowering the levels of contraceptives, antiretrovirals, and some anticoagulants. Review the patient’s medication list thoroughly.
• Using Streptomycin without auditory monitoring: Early hearing loss can be irreversible. Conduct baseline audiometry and repeat after two weeks.
• Stopping therapy early because of mild liver enzyme rise: Small, transient increases are common; continue with close monitoring unless enzymes rise > three times the upper limit.
• Prescribing Bedaquiline without ECG baseline: Missed QT prolongation can precipitate arrhythmia. Get a baseline ECG and repeat monthly.