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Blood Disorders

BLOOD CLOTS AND THE PILL - a patient's guide


There has been extensive debate about which type of contraceptive pills may or may not be associated with an increased risk of blood clots. This article offers an excellent overview and summary of the current evidence to help women with their choices.

blood clots and the oral contraceptive pill

What are blood clots?

Venous thrombosis is when a blood clot forms in the veins deep inside the muscle at the back of the lower leg. You may not notice anything different or you may notice that one of your calves is sore and swollen. If both legs are sore it is most unlikely to be because of a blood clot. Occasionally a piece of the blood clot can break off and travel to the heart and lungs. Then you might notice you are short of breath or you might cough up blood. One or two percent of people with a blood clot die from this complication.

Blood clots in an artery are different and very rare in young women. They can cause a heart attack or stroke.

Who is likely to get a blood clot?

If you are not on the pill and not pregnant you have a 1 in 30,000 chance of a blood clot in a leg vein.

If you are pregnant you have a 30 in 30,000 chance of a blood clot.

If you take a combined oral contraceptive pill containing oestrogen you have a 3 to 6 per 30,000 chance of a blood clot.

The risk of having a car accident or even an accident at home is much greater than the risk of using the pill.

Other things make you more likely to have a blood clot:

  • If you have ever had a blood clot before
  • If someone in your family has had a blood clot before - if your mother, father, sister or brother definitely had a blood clot before they were 55 years old you will be offered a blood test to check whether you are also at high risk
  • If you have injured your leg particularly if you are in a plaster cast
  • If you are in a wheelchair or bedridden or have had surgery recently
  • If you are overweight
  • Temporarily when you are on a long flight or get dehydrated
  • Some serious illnesses such as cancer

Is there more risk with some pills?

This is still being debated.

Blood clots in the veins are related to the oestrogen in the pill. The first generation pills had high doses of oestrogen and so had a high risk of blood clots. The dose was reduced for this reason in second generation pills and earlier studies showed venous blood clot rates of about 6 per 30,000. Third generation pills have the same amount of oestrogen as second generation pills but they have different progestogens, the other hormone in combined pills.

Third generation pills were developed to reduce unwanted side effects such as greasy skin and hair, and acne as well as reduce the tiny risk of clots in the arteries. Many women notice that they get less side effects on third generation pills. These pills do not alter the fats in the blood and while there is some evidence that they may decrease heart attacks the most recent study shows no difference.

The studies on blood clots reported in 1995 showed that second generation pills had a rate of 3 in 30,000 compared to third generation pills with a rate of 6 in 30,000. The third generation pills now had the same rate as the second generation pills a decade earlier while the second generation pills had a lower rate than before.

The question is whether this is a because of a difference in the pills or a difference in the people using them, or the way they were prescribed.

Third generation pills were very popular in New Zealand and the price was no different from the second generation pills. So 3 out of 4 women on a combined pill in New Zealand were using a third generation pill. This was partly because they were thought to be safer than the older pills.

It seems clear that if a woman had something that made it a little more risky for her to be on the pill, her doctor was likely to put her on a third generation pill. This particularly applied to older women in their 30s and 40s who have a higher risk of clots in both the veins and arteries.

Blood clots in the veins are more likely when you go on to the pill for the first time. People who are happy on the second generation pill tended to stay on it while people who had problems and people starting the pill for the first time were likely to be prescribed third generation pills. So there were more people who had risk factors for clotting in the group of women using third, than second generation pills.

It was found in the 1995 studies that women taking pills with only 20mcg of oestrogen had higher rates of clotting in the veins than those taking 30mcg and that as each new pill came onto the market it had a higher rate of blood clots. Both these findings suggest that there are differences in the women and the prescribing rather than differences in the pills themselves.

Only one explanation for the difference has been found. One group found a clotting factor difference between third and second generation pills. However this clotting factor seems to be associated with clotting in the arteries not in the veins.

Since the 1995 studies there has been much debate, reanalysis of those studies and new studies to try and eliminate some of the possible flaws in those studies. Most of these recent studies have shown little or no difference in the clotting rates.

A recent reanalysis of one of the 1995 studies taking better account of the lifelong oestrogen exposure of a woman showed no significant difference between the second and third generation pills and also that 20mcg pills had a lower rate than 30mcg pills. This removes the previously inexplicable finding relating to the dose of oestrogen and therefore seems more likely to be an accurate analysis.

Which pill should you use?

The New Zealand Ministry of Health recommends that any woman wanting to use a combined pill be asked about her own and her family's health so risk factors can be identified. Her blood pressure and weight should be checked. She should then be told of the 1995 studies and the differences identified in them of clotting in second and third generation pills.

The Ministry recommend that the doctor who prescribes a combined pill for a woman for the first time consider a second generation pill as long as the woman has no identified risk factors. If she gets side effects from the second generation pill she can change to a third generation pill. If she has been on a third generation pill already and has no risk factors she can continue that pill.

New Zealand and Norway are the only 2 countries in the world that now recommend doctors prescribe a particular type of pill because of blood clotting risks. Some countries like the United Kingdom recommend that as long as a woman is aware that second and third generation pills may have different clotting risks she can choose what pill she wants while other countries do not think there is a significant enough difference to make any recommendation.

There is little evidence to suggest that low dose pills containing 20mcg of oestrogen are significantly safer than 30mcg pills.

Which pills are which?

Second generation pills:

  • Levlen*
  • Microgynon 30
  • Monofeme*
  • Brevinor
  • Norimin*
  • Trifeme*
  • Triphasil
  • Triquilar*
  • Brevinor 1
  • Loette
  • Microgynon 20

Third generation pills:

  • Mercilon
  • Marvelon
  • Femodene
  • Minulet

*Fully subsidised pills

Diane 35 is best considered to have a risk of blood clotting similar to third generation pills.

What about the pill long term?

A 25 year study of women in the United Kingdom using the pill or condoms showed no difference in death rates. This is because there is a balance between women who die because of such illnesses as clots and breast cancer while on the pill and the protective effect of the pill in preventing women getting cancer of the ovary.

Getting help

Talk to your family doctor or go to a Family Planning clinic.

See also:

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