What drug is commonly used as first-line therapy for Giardia lamblia infection?

• A. Albendazole
• B. Ivermectin
• C. Metronidazole
• D. Nitazoxanide

Answer: (C)

Metronidazole is commonly used as first-line therapy for Giardia lamblia infection because it is a nitroimidazole with strong, well-established effectiveness against Giardia. It becomes activated inside the anaerobic parasite, forming reactive intermediates that damage DNA and other critical molecules, which kills the organism. It also penetrates well into the intestinal lumen and tissues, allowing it to address both luminal and tissue-associated forms of infection. The drug is easy to administer, widely available, and has a long history of success in treating giardiasis, supported by numerous clinical trials. While other options exist for special situations or intolerance, metronidazole remains the go-to first-line choice due to its proven efficacy, practicality, and cost-effectiveness.