<prompt> All of the following are true regarding penicillin except: March 2005Probenecid decreases it duration of actionAll penicillin are active against gram positive organismsSplits into 6-amino penicillanic acid by amidaseActs by inhibiting cell wall synthesis  <response>: Ans. A: Probenecid decreases it duration of action Drug interactions have been deliberately used to prolong the effect of penicillin, such as co-administering probenecid with penicillin prior to mass production of penicillin. Because penicillin was difficult to manufacture, it was wohwhile to find a way to reduce the amount required. Probenecid retards the excretion of penicillin, so a dose of penicillin persists longer when taken with it and it allowed patients to take less penicillin over a course of therapy. 