Hello and welcome, If you are using combined pills that contain ethinyl estradiol and a progesterone along with clarithromycin then there is a very small risk that the pill may be ineffective. The clarithromycin is generally cleared from the system in 36-48 hours. So it is better to begin the pills 2 days after stopping the drug. In that case you will be starting them on an odd day and not the first day of the period. So they will start providing protection 7 days after the first pill. The risk is very very small and mostly theoretical and most of us do not even consider this risk. But if you want to be on the safer side just follow the above schedule. Hope this satisfies your query. Thanks for using HCM. Feel free to ask any more questions that you may have. Dr Madhuri Bagde Consultant Obstetrician and Gynecologist