From the following list, select the drugs that should not be taken concurrently with penicillin

a. tetracycline.
b. metronidazole.
c. ciprofloxacin.
d. cephalosporin.
e. doxycycline
f. azithromcyin

a, e,f. Penicillin + a bacteriostatic antibiotic (e.g., tetracyclines, erythromycins) creates a bactericidal antibiotic requires that the bacteria be actively multiplying to be effective. If a bacteriostatic antibiotic is given concurrently, then the penicillin will not be effective.