Medical Definition of penicillin G
: the penicillin C16H18N2O4S that constitutes the principal or sole component of most commercial preparations and is used chiefly in the form of its stable salts (as the sodium salt C16H17N2NaO4S or potassium salt C16H17KN2O4S)—called also benzylpenicillin; see penicillin g benzathine, penicillin g procaine
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