Confused about onset effectiveness of depo provera (medroxyprogesterone)? some doctors say 1 month, 1 week, 2 weeks or effective immediately when on period. what is it?

Immediately when on: An intramuscular injection of 150 mg of Depo-Provera given "when on period" to an average weight/average BMI woman will usually prevent ovulation and become quite effective in preventing pregnancy "immediately". The onset of effectiveness is suspected to be a little more variable for some obese women and in very rare circumstances.

Timing matters: Possibility of pregnancy is not turned off like a switch. If you ovulate today, get depo today and have unprotected sex tonight, you may get pregnant.Depo or the pill works primarily by preventing ovulation.If you ovulate before it takes effect, your egg might encounter sperm and conceive.While this and other hormonals (pill) work quickly,many suggest you use a backup for up to a month.