I didn't gain a single lb on the IUD.I've had it off and on for 5 years.
It has 1/7 the amount of hormones as the pill. And the hormones go directly into the lining of the womb versus into the blood stream.
It acts as a contraceptive in two ways: it makes the mucus at the opening of the cervix much thicker, preventing sperm from getting through and it also makes the lining of the womb extremely thin, stopping implantation. In some women it prevents egg release (ovulation).
I don't get my period but I spot for a couple days once a month.