Do they give a patient zosyn (piperacillin and tazobactam) when allergc to penicilln?

Generally no. But it depends on the allergy and the indication. If the allergy to penicillin is mild and the indication is for a life threatening illness, then zosyn (piperacillin and tazobactam) (piperacillin/tazobactam) can be cautiously used.