To illustrate how much this can matter: In 1995, the UK... issued a warning that third generation oral contraceptive pills increased the risk of thrombosis by 100%.
What that meant was that the risk of getting thrombosis
-- potentially life-threatening blood clots in the legs or lungs
-- went from one in 7,000 for women taking second generation birth control pills to two in 7,000 for those taking the third generation variety.
Not a huge risk, then.
But a 100% risk increase sounds quite ominous.
The warning scared many women away from the pill (as psychologist Gerd Gigerenzer describes in his book Risk Savvy).
One result was an estimated 13,000 additional abortions the following year in England and Wales. Another was, ironically, a lot of thrombosis cases
-- pregnancies and abortions are much more likely to bring on the condition than does taking a third generation birth control pill.