, ▁or ▁di arr he a . ▁Gast ro int est inal ▁dis orders ▁or ▁inter pat ient ▁vari ability ▁in ▁or al ▁contra cept ive ▁absor ption ▁affect ing ▁eth in yl est rad i ol ▁ser um ▁levels ▁in ▁the ▁blood . ▁Women ▁with ▁men stru al ▁irregular ities ▁may ▁be ▁at ▁higher ▁risk ▁of ▁failure ▁and ▁should ▁be ▁ad vised ▁to ▁use ▁backup ▁contra ception ▁during ▁ant ib i otic ▁treatment ▁and ▁for ▁one ▁week ▁after ▁its ▁completion . ▁If ▁patient - specific ▁risk ▁factors ▁for ▁reduced ▁or al ▁contra cept ive ▁effic acy ▁are ▁sus pected , ▁backup ▁contra ception ▁is ▁recommended . ▁ ▁In ▁cases ▁where ▁ant ib iot ics ▁have ▁been ▁suggested ▁to ▁affect ▁the ▁efficiency ▁of ▁birth ▁control ▁p ills , ▁such ▁as ▁for ▁the ▁broad - spect rum ▁ant ib i otic ▁rif amp ic in , ▁these ▁cases ▁may ▁be ▁due ▁to ▁an ▁increase ▁in ▁the ▁activities ▁of ▁he p atic ▁li ver ▁en zym es ' ▁causing ▁increased ▁break down ▁of ▁the ▁pill ' s ▁active ▁ing red ients . ▁Effect s ▁on ▁the ▁int est inal ▁fl ora , ▁which ▁might ▁result ▁in ▁reduced ▁absor ption ▁of ▁est rog ens ▁in ▁the ▁colon , ▁have ▁also ▁been ▁suggested , ▁but ▁such ▁suggestions ▁have ▁been ▁incon clus ive ▁and ▁controvers ial . ▁C lin icians ▁have ▁recommended ▁that ▁extra ▁contra cept ive ▁measures ▁be ▁applied ▁during ▁ther ap ies ▁using ▁ant ib iot ics ▁that ▁are ▁sus pected ▁to ▁interact ▁with ▁or al