New contraceptive pills double risk of blood clots

Since its introduction in the 1960s, the oral contraceptive pill (OCP) has been linked to an increased risk of blood clots.

Now a Danish literature review confirms what has been suspected for years: that the so-called fourth-generation OCP doubles the risk of blood clots compared to second-generation OCPs.

The review was conducted by a medical student and a doctor at Aarhus University Hospital’s Department of Clinical Pharmacology.

They have gathered all the literature in the field and assessed its quality. This has enabled them to say with a high degree of certainty that the fourth-generation OCP is more dangerous in terms of an increased risk of blood clots than the second-generation OCP.

The study primarily consisted of a comparison of the two generations of pills, but the researchers recommend that people should, whenever possible, stick to the second generation pills.

Why do OCPs increase the blood clot risk?

The problem with contraceptives is that they contain the synthetic estrogen-like hormone Ethinyl estradiol.

This hormone increases the risk of blood clots because it increases the formation of clotting factors in the blood.

“On the one hand, the blood has to flow freely in the veins, but on the other hand, it should also stop a bleeding in case of an injury. It’s a delicate balance. OCPs alter this balance so that the blood will clot more easily, and thereby increases the risk of blood clots,” says Dr Birgitte Klindt Poulsen, who together with medical student Marlene Lunddal Krogh conducted the review.

The pills they studied are also known as combined oral contraceptive pills (COCP), since they also contain another hormone, called gestagen, which is similar to progesterone – a hormone that controls the female menstrual cycle.

Part of the reason the COCPs are grouped into generations is that they contain different types of gestagens.

The various chemical structures of gestagen cause slight alterations in the oestrogen, and that increases the risk of blood clots.

Poulsen points out that there are usually several factors that come into play when people get blood clots, but that oral contraceptives may well be “the last straw that breaks the camel’s back,” as she says.

No cause for panic

There is, however, no cause for wild panic if you’re using fourth-generation OCPs.

• The absolute risk of developing blood clots for 20-year-old women who do not use OCPs is 1-2 cases per 10,000.
• With second-generation OCPs, the risk increases three-fold to 3-6 cases per 10,000.
• With fourth-generation OCPs, the risk increases six-fold to 6-12 cases per 10,000.

The risk of blood clots also increases with age. Furthermore, the risk is higher if you are overweight or have a family history of spontaneous blood clots.

For this reason, it can sometimes be difficult to determine whether it’s just the OCPs that caused the blood clot.

“I think the pill is good because it can ensure that you’re in control concerning the risk of getting pregnant,” says Poulsen. “But I think you and your doctor should consider whether you are already at risk for blood clots before you start using them.”

The Pill will probably never be completely safe

The risk of blood clots is 50 percent higher in the first year after you start using OCPs. The risk is then reduced, but it never reaches the levels of women who do not use OCPs.

The Pill has become safer to use over the years, but it’s still not entirely without risk:

“It’s unlikely that we’ll ever see an OCP that’s entirely safe,” says Poulsen. “This is because the Pill only works by virtue of adding some hormones that alter the blood’s delicate clotting abilities.”

In the past year, second-generation OCPs have been the first choice for Danish doctors.

If it later turns out that the patient is experiencing strong side effects from these pills, then other generations will be considered.

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Read the Danish version of this article at videnskab.dk

Translated by: Dann Vinther

External links

• Birgitte Klindt Poulsen's profile

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