However, these pills caused irregular bleeding and proved less effective in preventing pregnancy, so their use was limited. By the mid-1970s, the birth control pills contained less than 50 micrograms of estrogen and between 0.15 and 2.5 milligrams of progestin (Snider).
In 1982, a biphasic pill was introduced, and two years later three triphasic pills (Snider). These are low-dose pills in which the ration or estrogen to progestin changes during the 21 days the pill is taken. In 1988, at the urging of the FDA, the three manufacturers still making high-dose estrogen birth control pills voluntarily withdrew them from the market. Early fears of blood clots, he
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