Newest Birth Control Pills May Cause Blood Clots

September 24, 1999 -- The latest generation of birth control pills, which were introduced in the 1980s and early 1990s, may raise a woman's risk of blood clots even more than earlier oral contraceptives, according to a report from Denmark.

From 1977 to 1993, a team of researchers led by Dr. Lene Mellemkjaer, of the Danish Cancer Society, tracked hospital admissions for venous thromboembolism, a group of disorders that includes pulmonary embolism (clots in the lung), and deep venous thrombosis (most often clots in large veins in the legs).

The study authors found that for both men and women aged 15 to 49, the number of cases of venous thromboembolism was fairly steady from 1977 to 1988. In the period from 1989 to 1993, however, the hospitalization rate for women was more than 16% higher.

The increase in hospitalizations coincided with increasing use of the newer Birth Control drugs, they report. The so-called third generation pills were used by just 0.2% of Danish women who took oral contraceptives in 1984, but that percentage increased to 17% in 1988, 40% in 1990, and 66% in 1993.

``Our study gives support to the hypothesis that third generation birth control pills increase the risk of venous thromboembolism to a larger extent than second generation birth control pills,'' Mellemkjaer told Reuters Health. However, the Danish researcher stressed that the study could not prove that the newer pills were to blame.

However, in the report, the authors note that earlier studies have also suggested that the newer contraceptives increase the risk of blood clots more than second generation birth control pills. Most birth control pills contain either a combination of the hormones estrogen and progestin or progestin alone. According to Mellemkjaer, the main difference between second and third generation birth control pills is in the level of progestin.

``The increased risk of venous thrombosis with third generation pills is real and measurable, but it is also small in absolute terms, although greatest in women starting the Pill,'' Dr. Paul A. O'Brien, of the Parkside Health NHS Trust in London, UK, writes in an accompanying editorial. He states that second generation birth control pills should be ``the first choice.''

However, some women may be willing to accept the small additional risk of blood clots in exchange for the potential benefits of the third generation pills, such as reduced acne, according to O'Brien.

``It is not that third generation contraceptives are unsafe -- it is just that we have something safer,'' he concludes.

SOURCE: British Medical Journal 1999;319:795-796, 820-821.