Methicillin sodium anhydrous is a sodium salt of methicillin (methicillin). Methicillin is an antibiotic formerly used in the treatment of bacterial infections caused by organisms of the genus Staphylococcus. Methicillin is a semisynthetic derivative of penicillin. It was first produced in the late 1950s and was developed as a type of antibiotic called penicillinase-resistant penicillin—it contained a modification to the original penicillin structure that made it resistant to a bacterial enzyme called penicillinase (beta-lactamase). Compared to other penicillins that face antimicrobial resistance due to β-lactamase, it is less active, can be administered only parenterally, and has a higher frequency of interstitial nephritis, an otherwise-rare adverse effect of penicillins. However, the selection of meticillin depended on the outcome of susceptibility testing of the sampled infection, and since it is no longer produced, it is also not routinely tested for anymore.

PENICILLIN V HYDRABAMINE, a semi-synthetic antibiotic, is used for the treatment of mild to moderately severe infections (e.g. dental infection, infections in the heart, middle ear infections, rheumatic fever, scarlet fever, skin infections, upper and lower respiratory tract infections) caused by susceptible, usually gram-positive, organisms.

PENICILLIN G HYDRABAMINE, a semi-synthetic antibiotic, is used for the treatment of infections caused by susceptible, usually gram-positive, organisms.

ALMECILLIN (also known as penicillin O) is an antibiotic that can be safely substituted for penicillin G in instances of hypersensitivity reactions to the latter.