Megapen 500mg Injection is a combination antibiotic formulation containing Ampicillin 250mg and Cloxacillin 250mg per dose, designed for the treatment of a broad spectrum of bacterial infections. Ampicillin, a beta-lactam antibiotic, exhibits bactericidal activity by inhibiting bacterial cell wall synthesis, primarily effective against Gram-positive and some Gram-negative organisms. Cloxacillin, a penicillinase-resistant penicillin, targets penicillinase-producing staphylococci, thereby enhancing the spectrum of activity against resistant strains. This synergistic combination is indicated for infections caused by susceptible strains, including respiratory tract infections, skin and soft tissue infections, bone and joint infections, urinary tract infections, and septicemia. Megapen 500mg Injection is administered via intramuscular or intravenous routes, with dosing adjusted according to the severity of infection, patient age, and renal function. It is contraindicated in patients with hypersensitivity to penicillins or cephalosporins. Caution is advised in individuals with a history of allergic reactions to beta-lactam antibiotics. Common adverse effects include hypersensitivity reactions, gastrointestinal disturbances, and local injection site reactions. Proper microbiological diagnosis and susceptibility testing are recommended to ensure appropriate use and to minimize the development of resistance. Megapen 500mg Injection should be used under strict medical supervision to optimize therapeutic outcomes.