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Penicillin is an antibiotic that disturbs the cell wall synthesis in bacteria. It was initially derived from the mold Penicillium rubrum. Since then, a number of naturally occurring penicillins have been derived e.g. penicillin G (benzylpenicillin) and penicillin V (phenoxymethylpenicillin).

Other antibiotics obtained from penicillins include amoxicillin, ampicillin, flucloxacillin. These are classed as semisynthetic penicillins.

Penicillins are bactericidal and mainly active against gram-positive bacteria.

The most important side effect of penicillins is hypersensitivity, such as rashes or anaphylaxis. Patients who are allergic to one penicillin will be allergic to all.