text: There were four named penicillins at the time penicillin V was discovered (penicillins I, II, III, IV), however, Penicillin V was named "V" for Vertraulich (German for confidential); it was not named for the Roman numeral "5".
  Penicillin VK is the potassium salt of penicillin V (K is the chemical symbol for potassium).