Benzylpenicillin

The naturally occurring penicillins constitute a group of closely related acids whose sodium, potassium and other salts can be used for therapeutic purposes. The sodium or potassium salt of one of these acids, penicillin G is officially known as benzylpenicillin, but is customarily termed crystalline penicillin or soluble penicillin. It is rapidly absorbed following intramuscular injection and is excreted by the kidneys within a few hours. Half a million units twice daily will suffice for most infections due to sensitive organisms. By prescribing larger doses, such as 2-6 million units of benzylpenicillin intramuscularly, very high blood levels of penicillin can be produced and therapeutic concentrations can be achieved within deep-seated or walled –off foci of infection, as occur in subacute bacterial endocarditis, or lung abscess. Quantities of this order are painful to inject and therefore must not be administered unnecessarily. Probenecid given concurrently with benzyl penicillin will raise the blood level of penicillin by delaying its excretion by the kidney and hence reduce the size, number and pain of the injections. The dose of probenecid is 2g.