af c ill in ▁is ▁indicated ▁in ▁the ▁treatment ▁of ▁st ap hy loc oc cal ▁in fe ctions , ▁except ▁those ▁caused ▁by ▁MR SA . ▁ ▁U . S . ▁clin ical ▁practice ▁gu idel ines ▁recommend ▁either ▁na fc ill in ▁or ▁ox ac ill in ▁as ▁the ▁first - line ▁treatment ▁of ▁choice ▁for ▁st ap hy loc oc cal ▁end oc ard itis ▁in ▁patients ▁without ▁artificial ▁heart ▁val ves . ▁ ▁Side - effect s ▁As ▁with ▁all ▁pen ic ill ins , ▁serious ▁life - th re aten ing ▁all erg ic ▁re actions ▁can ▁occur . ▁ ▁M ilder ▁side - effect s ▁include : ▁ ▁Hyp ok alem ia ▁ ▁N ause a ▁and ▁vom iting ▁ ▁Di arr he a , ▁often ▁due ▁to ▁supp ression ▁of ▁normal ▁g astro int est inal ▁b acter ia , ▁which , ▁on ▁occasion , ▁leads ▁to ▁a ▁more ▁serious ▁super - in fection ▁with ▁an ▁organ ism ▁like ▁Clo str id ium ▁diff icile ▁ ▁Ab dom inal ▁pain ▁ ▁Ye ast ▁in fe ctions ▁( th r ush ) ▁affect ing ▁the ▁mouth ▁and ▁tongue ▁or ▁v ag ina ▁ ▁Agr an ul oc yt osis , ▁neut rop enia ▁ ▁Inter actions ▁There ▁is ▁evidence ▁that ▁na fc ill in ▁indu ces ▁cy to chrome ▁P - 4 5 0 ▁en zym es , ▁specifically ▁C Y P 2 C 9 . ▁Several ▁dru gs ▁with ▁a ▁narrow ▁th era pe ut ic