A 2018 systematic review of 26 studies, including 5 randomized controlled trials and 21 observational studies, found that the overall evidence showed no association between progestogen-only birth control and depression. The progestins assessed included depot medroxyprogesterone acetate, levonorgestrel-containing contraceptive implants and intrauterine devices, and progestogen-only birth control pills. Findings of large observational studies are mixed due to prominent confounding factors, but overall show no association of hormonal birth control with depression. Randomized controlled trials typically do not find clinically significant influences of hormonal birth control on mood. Reviews from before 1980 reported a high incidence of adverse mood effects with combined birth control pills. However, doses of estrogens and progestogens in birth control pills before 1980 were considerably higher than those used today, and these doses frequently caused unpleasant side effects that may have unfavorably influenced mood.