Ang injectable contraceptive usually kailangan ng 7 days bago fully effective, so yes, kailangan mo talaga ng backup like condom during that first week, better safe than sorry talaga! One of my mum friends got her Depo shot and she and her hubby were extra careful those first days lang, kaya okay naman. Pero honestly, best to confirm with your OB kung anong specific injection you're getting, kasi baka may different guidelines depending sa brand or your situation, para sure ka talaga!