The year 1961 saw the first reports of increased risk of blood clots (venous thrombosis) from use of oral contraceptives. Since then, we have seen the amount of estrogen in the pills reduced over the years. While the initial pills had 100 micrograms of estrogen, the first reduction was to 50 micrograms and now 35, 30 and even 20 micrograms.
As the Society of Obstetricians and Gynecologists of Canada points out, there are two types of estrogen used in combined contraceptive pills: ethinyl estradiol and mestranol. Mestranol is a “pro-drug” that is converted in the body to ethinyl estradiol. Ehinyl estradiol is now used in the majority of the combined oral contraceptive pills. Several different progestins are used in combined OCPs. They are all 19- nortestosterone derivatives, including the newer third generation progestins, desogestrel, gestodene and norgestimate.
In British Medical Journal are two studies looking at the risk of such blood clots and the types of progestin and the dose of estrogen. An important point to remember is that blood clots can happen to anyone, regardless of whether they take the Pill. The overall risk of a blood clot in a non-Pill user is 3.01 per 10,000. The absolute risk in a current user is 6.29 per 10,000 users. So overall, the absolute risk is low. As well, the risk is far smaller than the risk of a blood clot in pregnancy.
Contraceptive pills also contain progestins. Two of the progestins used are often prescribed for acne. The first is cyproterone acetate. Preparations containing drosperinone are thought to cause less fluid retention. As the authors point out, a recent Cochrane review concluded that only minor differences were found in the effectiveness of preparations containing different progestins for the treatment of acne, seborrhea or mild hirsutism. Also with regard to weight gain while using the different OCP, no major differences were found. One can argue that if all the pills are equally effective, then the lowest-risk pills should be used, even though the absolute risks of less than 1/1000 blood clots in users is small. Any lowering of risk no matter how small should be considered important. The study found three important points. Firstly, the risk of a clot decreases with decreasing doses of estrogens. Secondly, the risk of a clot decreases with increasing use. To that end, the risk of a clot in users of less than a year was 4.17/10,000, 1-4 years 2.98/10,000 and more than 4 years 2.76/10,000.
Thirdly, both studies confirmed that the kind of progestin used will influence the risk. Here, the authors use the concept of “relative risk,” meaning compared with the baseline risk. They are not absolute numbers. The authors found that the newer generation progestins conferred more of a relative risk. The users with cyproterone acetate and desogestrel had about a seven-fold increased risk over non-users and risperidone had a six-fold increased risk. Progestogen only pills and hormone-releasing IUD were not associated with any risk of venous thrombosis.
Both studies conclude that despite the fact that the absolute risk of a blood clot is fairly low, oral contraceptives do have an impact on thrombosis occurrence and some pills confer less of a risk than others. The absolute risk of venous thrombosis with use of any types of combined OCP is less than 1 in 1,000 user years. The choice of oral contraceptive is a decision between patient and health care provider. The choice of oral contraceptive is a decision between patient and health care provider