How do you know the depo shot starts working in your system? Is there any particular symptoms that would let you know that it’s working?

I took my very first shot in April 18 and as of now, there really isn’t anything that tells or shows me that it’s finally working in my system, but a creamy discharge for a couple of days now and backache here and then.

I was on birth control pills for a year before I changed to depo. I gained weight, period cramps disappeared and my period would always be light. I knew then that it started working in my system. My question is, would it be the same on depo shot?

I know depo shots are up to 97% effective, but it still makes me anxious.