Megapen 1 gm Injection is a combination antibiotic formulation containing Ampicillin 500 mg and Cloxacillin 500 mg, designed for the treatment of a wide range of bacterial infections. Ampicillin, a broad-spectrum penicillin antibiotic, exhibits bactericidal activity by inhibiting bacterial cell wall synthesis, effective against both Gram-positive and certain Gram-negative organisms. Cloxacillin, a penicillinase-resistant penicillin, specifically targets penicillinase-producing Staphylococcus aureus strains, thereby enhancing the spectrum of activity against resistant Gram-positive bacteria. Megapen 1 gm Injection is indicated for infections caused by susceptible strains, including respiratory tract infections, skin and soft tissue infections, bone and joint infections, septicemia, and urinary tract infections. The combination therapy provides a synergistic effect, improving clinical outcomes in mixed infections or those caused by beta-lactamase producing organisms. The injection is administered via intramuscular or intravenous routes, with dosage and duration tailored according to the severity and type of infection, as well as patient-specific factors such as age and renal function. Caution is advised in patients with a history of penicillin allergy. Common adverse effects may include hypersensitivity reactions, gastrointestinal disturbances, and local injection site reactions. Megapen 1 gm Injection should be used under strict medical supervision to ensure appropriate use and to minimize the development of antibiotic resistance.