The first-line drug that is typically used to treat a Clostridium difficile infection is vancomycin. Vancomycin is an antibiotic that is effective against C. difficile and is often used as the first-line treatment for mild to moderate cases of the infection. In more severe cases, other antibiotics such as fidaxomicin or metronidazole may be used, but vancomycin is generally considered the first-line treatment. It's important to note that the use of antibiotics can sometimes lead to the development of antibiotic-resistant strains of C. difficile, so it's important to use antibiotics only when necessary and to take them exactly as prescribed.