pill injection?

Louise
So recently I've got a few big things planned, nights away etc for the next few months n I was thinking I should probably start some sort of contraception... 
My sister recently had the pill injection n has had it in the past, she said it was great, full stopped all her periods although it made her gain weight. But I'm not bothered about any of that I need to know if this pill injection is full trustable. 
Any women out there that have tried this and had unprotected sex please let me know - as I'll probably still stress even if I don't need to. 

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