The official take from the drug company that produces it says it is effectively immediately, for 3 months:
http://www.depoprovera.com/timeline_whattoexpect.asp
You may also find this of interest, from these very boards:
http://www.able2know.com/forums/viewtopic.php?t=9359&postdays=0&postorder=asc&start=0
Dragon, don't worry too much. I happen to work in the field of pharmaceautical research. I have seen several studies on Depo-pravera. For the most part, you are in no danger of getting pregnant. You should be worried about STD's, so make sure that you and your sexual partner are checked regularly.
Now, mind you, occasionally someone does get pregnant while using any number of birth control devices. It isn't a perfect science. The most common side effects of Depo are mood swings. Some women tend to get kinda grouchy when they take the shot. Basically, Depo screws around with your reproductive tract. If and when you stop taking the drug, you may experience irregular menstruation for up to several months afterward.
I think it's smart of you to use 2 forms of contraception at the same time.
The pill usually takes a month to really kick in, but the shot should take only hours. Good luck!
What I don't understand is why all these doctors are administering Deprovera shots without giving their patients answers to these very basic questions about the drug. What's going on in the medical profession?
Why aren't patients asking their doctors these types of questions BEFORE getting the shots? Why aren't they insisting it be explained until they understand?
I was told the shot was *immediately* effective. Me and my wife had unprotected sex the first day (no pulling out either) and she's fine.
How's it goin? Umm......ok, I am new here and I have some questions. LOL!! 
On September 11, I went to the doctor and got the Deprovera shot. Then I had sexual intercourse on Sept. 13 and the condom broke. 
Now, my questions are, after you get the shot how long does it take to take effect, to prevent pregnancy? 
Is there a possibility for me to get/be pregnant? Thank you, I really appreciated it. 
I heard its effective immediately if you get the shot while having your period but if you get it and you dont have your period then you have to wait a week, is that true? I went to the website and read the info. but I am still confused. Sorry, I am asking too many question is my first time using the shot. Thank you. 
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