Could you tell me if someone was to double up on loestrin 20 (the pill), would it cause serious illness?

Possibly: Big question is, why you want to do that? Birth control pills are prescribed at the lowest effective hormone dose that stops ovaries from releasing eggs. No eggs = no pregnancy. The higher the level of hormones, the higher the risks of complications like overgrowth of the uterus lining, or blood clots in your legs, lung, heart, or brain (esp. If you smoke). Talk to your dr about what you want.