For long term use, my girl was on birth control for about 4 years.
I don't know what dosage, and I know most birth control is now a pretty low dose of estrogen, but to be effective, it seems it would have to be a high enough dose to shut down most of your natural estrogen levels.
It's normally enough to make a girls skin clear, put on a little weight, bloat and increase breast size.
Coming off them, after being close, or being shut or close to it for 4 years seems like it could cause some long term effects on hormones.