Broad spectrum antibiotics , such as ampicillin and doxycycline , may cause problems `` by impairing the bacterial flora responsible for recycling ethinylestradiol from the large bowel '' ( BNF 2003 ) . In addition cautions are given about broad spectrum antibiotics , such as ampicillin and doxycycline , which may cause problems `` by impairing the bacterial flora responsible for recycling ethinylestradiol from the large bowel '' ( BNF 2003 ) .