I have been finding some extremely conflicting information regarding the use of Depo-Provera (a progesterone contraceptive injection) and doxycycline (a tetracycline antibiotic). The prescribing information pamphlet for Depo-Provera indicates that enzyme-inducing drugs may decrease its effectiveness. The prescribing information for doxcycline indicates that it may decrease the effectiveness of oral contraceptives (i.e. birth control pills). Because Depo-Provera is injected, it bypasses gastrointestinal absorption. The doctor that prescribed me the antibiotic advised me to use back up birth control as the information regarding these interactions is conflicting. Two pharmacists, however, advised me that these two drugs do not interact and that back up methods are not necessary. Another pharmacist stated these these drugs can affect one another. I have even consulted medical journals and the majority of these journals indicate that TB antibiotics are the only ones known to interact with birth control. I am very concerned about this because I honestly forgot what my doctor recommended and engaged in unprotected intercourse while taking the antibiotics. The pharmacist I talked to told me that taking Plan B would not be necessary because antibiotics only affect birth control pills (not the shot or IUD) due to a disruption of bacteria in the stomach/intestines. I would greatly appreciate advice from anyone who has gotten information about this. Please explain the reason behind your answer.