The oral contraceptive pill, colloquially called "the pill," is made up of sex hormones related to estrogen and progesterone. These hormones are combined in different proportions, depending on the type and brand of pill that you take. Normally, estrogen and progesterone follow a cycle that repeats each month, affecting both the reproductive system and the brain.
The first contraceptive pill went on the market in August of 1960, and over a million American women were using it just one year later. The manufacturers of the pill downplayed the possible side effects of the pill, but in the years since its inception, women have blamed just about everything on it.
One theory is that the pill is linked to depression, based on the anecdotal evidence that many women report feeling "down" while on hormonal contraceptives. After all, messing with hormones and keeping them relatively stable for most of the month (rather than going slightly up or down, as is naturally the case) might arguably have an effect on the brain. Research, however, is split on whether the theory holds water.
Studies That Support the Connection
Some studies that have tested the theory have found that there may be a link between the pill and depression. For example, a 2005 study done by Australia’s Monash University looked at the depression symptom scores of women who used oral contraceptives and those who did not. None of the women had a clinical history of depression, and they had not been on medication that could act as an anti-depressant in the twelve months prior to the study. Researchers found that women on "the pill" averaged 17.6 on the depression rating scale, whereas women who did not use contraceptives averaged only 9.8. The researchers concluded that oral contraceptive pills could cause depression in vulnerable women.
Studies That Do Not Support the Connection
Other studies have shown that not only doesn't the pill cause depression, it may even improve symptoms of depression. One of these was another 2005 study, this one presented at the American Psychiatric Association's annual meeting by researchers in the perinatal and reproductive psychiatry clinical research program at Harvard Medical School. In this study, researchers followed a small sample of women (17) who were struggling with depression at a specific point in their menstrual cycles — before their menstrual periods. Although these women had all taken antidepressant medications for at least two months before the study began, they did not suffer from any depression except for PMS. They were then put on Yasmin, a birth control pill containing estrogen and progestin hormones that is taken for three weeks of the menstrual cycle. (During the fourth week of the cycle, women take a placebo pill.)
Two months into the study, researchers found that their scores on a standardized test showed an 80% drop in depression and a 40% drop in the mood swings of PMS. Based on this data, they hypothesized that the pill stabilized the hormone levels in the brain, which minimized the mood swings and depression that these women had experienced.
Web MD. "Birth Control Pill May Relieve PMS Depression." https://www.webmd.com/sex/birth-control/news/20050525/birth-control-pill-relieve-pms-depression
Aphrodite Women's Health. "Is The Pill Playing Havoc With Your Mental Health?" https://www.aphroditewomenshealth.com/news/hormones_depression.shtml
HuffPost Living. "Is Your Birth Control Pill Driving You Bananas?" https://www.huffingtonpost.com/doug-bremner/is-your-birth-control-pil_b_99675.html