Which of the following drugs is not likely to cause Pulmonary fibrosis?

Correct Answer: C. Metformin

Metformin is a biguanide antidiabetic agent that works primarily through hepatic glucose production inhibition and improved peripheral insulin sensitivity. Critically, metformin has no known association with pulmonary fibrosis in clinical practice or pharmacovigilance databases, even with long-term use in Indian diabetic populations. The drug is metabolically inert in the lungs and does not undergo bioactivation to toxic metabolites that could trigger pneumonitis or fibrotic cascades. Unlike the other options, metformin lacks the structural features (e.g., bleomycin's DNA-binding capability, nitrofurantoin's nitro group reactivity, methotrexate's antimetabolite mechanism) that predispose to pulmonary toxicity. This question tests recognition that not all drugs cause pulmonary fibrosis—a critical distinction in pharmacovigilance. Metformin remains the safest oral antidiabetic for long-term use without pulmonary risk, making it the correct answer to a "not likely to cause" stem.

Why the other options are wrong

A. Nitrofurantoin — Nitrofurantoin is a well-established cause of acute and chronic pulmonary fibrosis through nitro-group-mediated oxidative stress and immune-complex deposition in the lungs. Acute nitrofurantoin pneumonitis can progress to irreversible pulmonary fibrosis, especially with prolonged use in Indian patients with recurrent UTIs. This is a classic pharmacology board answer. B. Bleomycin — Bleomycin is a chemotherapeutic agent with dose-dependent pulmonary fibrosis as its most serious toxicity. The cumulative dose threshold (~400 units) triggers bleomycin-induced pulmonary fibrosis (BIPF) through direct DNA damage and fibroblast activation. This is the prototype drug for chemotherapy-related pulmonary toxicity in Indian oncology practice. D. Methotrexate — Methotrexate causes methotrexate-induced pulmonary disease (MTPD), including acute pneumonitis and chronic pulmonary fibrosis, through antimetabolite-mediated immune activation and epithelial injury. Risk increases with cumulative dose, especially in Indian rheumatology patients on long-term MTX for RA or SLE.

High-Yield Facts

Mnemonics

CHAMP-B for Pulmonary Fibrosis Drugs Chemotherapy (bleomycin, busulfan), Hypoglycemics (NOT metformin), Antibiotics (nitrofurantoin, amiodarone), Methotrexate, Procainamide, Busulfan. Metformin is the exception—it does NOT cause PF. Memory Hook: 'Metformin = Metabolically Inert' Metformin is filtered unchanged by kidneys and does not undergo hepatic bioactivation. No toxic metabolites → no pulmonary toxicity. Use this when differentiating antidiabetics.

NBE Trap

NBE pairs metformin with other drugs in a "not likely to cause" stem to test whether students reflexively associate all chronic-use drugs with pulmonary toxicity. The trap is assuming that because nitrofurantoin, bleomycin, and methotrexate are well-known PF culprits, metformin must also be—when in fact metformin is pharmacologically inert in the lungs.

Clinical Pearl

In Indian primary care, metformin is the first-line oral agent for type 2 diabetes precisely because of its excellent long-term safety profile—including zero pulmonary risk. Patients on metformin for 10+ years show no increased incidence of interstitial lung disease, unlike those on nitrofurantoin for chronic UTI prophylaxis, where pulmonary fibrosis is a recognized complication.

_Reference: KD Tripathi Ch. 49 (Antidiabetic Drugs); Harrison Ch. 246 (Drug-Induced Lung Disease)_