I'm pretty sure it has more to do with the stress your body is under when on IVs that causes the delay. The timing of a period is determined by when you ovulate. It you're under stress (or sick) ovulation is delayed. It may have to do somewhat which the chemicals in the drugs, but not as much as you might think.
I've always been regular as clockwork which has made birth control practically a non issue for us. Incidentally, I have an IUD, and my husband recently underwent a vasectomy.
There are so many variables in our health as CFers, particularly when it comes to reproductive issues. I strongly encourage sexually active CFers to be METICULOUS about birth control and overall compliance with their healthcare regime.