All birth control pills require a prescription from a healthcare provider. Combination birth control pills are available in 21-day packs, with 21 active tablets, or 28-day packs with 21 active tablets, and 7 inactive tablets. Extended-cycle pills such as Seasonique contain 84 active tablets and 7 inactive tablets. The inactive pills are included to help the woman remember when she should start a new pack of pills. If the inactive pills are missed, there is no chance this would result in a pregnancy. However, it is important to start the new pack and active pills on the correct day.
Low levels of serotonin , a neurotransmitter in the brain, have been linked to depression . High levels of estrogen, as in first-generation COCPs, and progestin, as in some progestin-only contraceptives, have been shown to lower the brain serotonin levels by increasing the concentration of a brain enzyme that reduces serotonin. This observation, along with some small research studies  have inspired speculation that the pill causes depression. In 2016, a large Danish study of one million women showed that use of COCPs, especially among adolescents, was associated with a statistically significantly increased risk of subsequent depression, although the sizes of the effects are small (for example, % of the women who took any form of oral birth control were prescribed anti-depressants for the first time, compared to % of women in the control group). 
Hormonal contraceptives (the pill, the patch, and the vaginal ring) all contain a small amount of man-made estrogen and progestin hormones. These hormones work to inhibit the body's natural cyclical hormones to prevent pregnancy. Pregnancy is prevented by a combination of factors. The hormonal contraceptive usually stops the body from ovulating . Hormonal contraceptives also change the cervical mucus to make it difficult for the sperm to go through the cervix and find an egg. Hormonal contraceptives can also prevent pregnancy by changing the lining of the womb so it's unlikely the fertilized egg will be implanted.