Metronidazole is included in some regimens to improve coverage for anaerobic bacteria.
Anaerobes are of relatively greater importance in patients with severe PID and metronidazole may be discontinued in those patients with mild or moderate PID who are unable to tolerate it.

500 mg i.m once

Replacing intramuscular ceftriaxone with an oral cephalosporin (e.g. cefixime) is            not recommended because there is no clinical trial evidence to support its use, and tissue levels are likely to be lower
which might impact on efficacy. Reports of decreasing susceptibility of N. gonorrhoeae to cephalosporins also supports the use of parenteral based regimens when gonococcal PID is suspected (to maximise tissue levels and overcome low level resistance)