I took plan b twice in the same cycle will it still be effective ?also i have to take a few shots in the next few days one of them is penicillin, will

Effective prevention: however, it may cause heavy or irregular bleeding/cramping. It should not interfere with antibiotic therapy.

Yes: Plan B can work in one of three ways depending on where you are in your cycle. It can prevent or delay ovulation, can interfere with the fertilization of the egg or may prevent implantation by altering the lining of the uterus. If you take Plan B One-Step within 24 hours, it is about 95% effective. The only known antibiotic to interfere with BCP is Rifampin.