Drug interactions, decreased effectiveness of birth control...?

I'm sick as a dog... I have an antibiotic, but it has a drug interaction warning that says that it can lessen the effectiveness of BC pills... and I understand that they say in those circumstances that you're re-dosing your body with those hormones as you take a pill every day, so just use back-up contraceptives while you're on the antibiotic... but what if you're on the Depo shot? Will I have to continue using backup until I go for my next shot? How does this work?