Could you explain the reasons for any changes in serum potassium when
patients are taking penicillin?  thank you

Well, penicillin can come in the form of penicillin G potassium, for
example, so if that is what you are using, then that would definitely
cause hyperkalemia. The only other things I can think of is that the
acidic carboxyl group and the metabolite penicilloic acid causes
acidosis, and the kidney compensates for acidosis by the H+/K+
antiporter, thus also causing hyperkalemia.

"Hypokalemia may develop during therapy with certain antibiotics.
Carbenicillin or other penicillin-like antibiotics exist as sodium or
potassium salts of impermeant anions and promote kaliuresis because
they increase net sodium excretion and because of an impermeant anion
effect. Amphotericin B increases the permeability of luminal membranes
to potassium and therefore promotes potassium secretion. Gentamicin
produces potassium losses by unknown mechanisms."

http://www.nursingceu.com/NCEU/courses/hypohyper/

Thank you for the comments pertaining to this question.  Though not
designated "answers", the comments were extremely helpful and prompt.

Elizabeth.