I'll just go ahead and say, serious side effects from birth control pills are very rare. The only real complication is caused by the fact that some birth control pills have estrogen. This is actually especially dangerous for people who smoke. This is because smoking constricts blood vessels, and the reduced blood flow is dangerous along with a hormonal birth control pill that stops ovulation.
These pills have a man-made form of your hormone estrogen!
As you already know, your menstrual cycle prepares for a potential pregnancy, by using hormones to prepare the lining of the uterus…so that if one of your eggs gets fertilized, it has a nice pillowy lining to use!
Ok, you might be wondering, if estrogen rises before ovulation and helps with your ovulation and thickening of the uterus for pregnancy, then how the heck is putting more estrogen in your body through a pill supposed to prevent pregnancy? Wouldn't taking an estrogen pill actually make you more likely to get pregnant?
Well, basically, because it suppresses something else!
Namely, it suppresses your follicle-stimulating hormone and luteinizing hormones, which are the hormones that help release a mature egg and also thicken the uterus lining.
So you have to have those two, FSH and LH, in order for ovulation to happen. And so if you block those, you block pregnancy! Estrogen, as well as progesterone, are used in birth control pills because they are able to block FSH and LH successfully.
Well…for one thing, ignore the people who say it's not natural to suppress ovulation and use medication that prevents pregnancy.
You want to know why? Well, women's bodies suppress this process NATURALLY when they're pregnant or nursing, so it's definitely not unnatural for their bodies to be suppressing FSH and LH! And in fact, estrogen birth control is very helpful for women who have reproductive conditions such as endometriosis (or even just really, really bad menstrual cramps)!!
Yet, you may wonder whether it's safe to use for a long "period" of time. The answer is: you may experience side effects, just as a result of altering hormone levels. Not everybody experiences side effects, but if you do, spotting between periods while your body adjust, feeling sick or getting a headache, or especially mood swings, are all normal. Sometimes they'll go away after the body does adjust, and for some people they won't.
However, I will say that there is an increased risk of breast cancer and cervical cancer from both estrogen and progestin based pills. For example, there was a 1996 review done of 54 studies, showing that women who took birth control pills had about a 20% higher risk of breast cancer, but that returned back to normal 10 years after they stopped birth control. But what you also have to remember here is, birth control pills in 1996 and before are not birth control pills today.
Today we have a lot of progestin-only pills, which are less associated with breast cancer and cervical cancer risk. These are called mini-pills, and while they have the same common side effects, are less likely to stimulate the growth of cancer cells.