Blood clots and HRT
BMJ Group Medical Reference
Blood clots are one of the most serious problems linked to taking hormone replacement therapy (HRT), but the increased risk is very small for most women.
When you get clots in your blood, doctors say you have thromboembolic disease. Although it isn't dangerous in itself, it can become life-threatening if a clot travels through your blood and blocks an important blood vessel. If a clot stops blood getting to your lungs, it is called a pulmonary embolism, which can kill you.
Researchers have found that women who take HRT are twice as likely to develop thromboembolic disease as women who don't take HRT. However, the risk of thromboembolic disease is small to begin with, so the increased risk for women who do use HRT is still not very big. In one large study: 
Over a period of about five years, less than 1 in 100 women taking HRT got a blood clot in their lungs
This was about twice the number of women who got this type of blood clot when taking a dummy treatment (a placebo) for comparison.
A big review of the studies found that the risk of blood clots was increased for women taking combined HRT tablets (oestrogen and progestogen) and for women taking oestrogen-only HRT tablets. But women who took HRT in the form of patches had no increased risk of blood clots.
If you've had blood clots before, you should let your doctor know and talk about whether HRT is suitable for you.
A placebo is a 'pretend' or dummy treatment that contains no active substances. A placebo is often given to half the people taking part in medical research trials, for comparison with the 'real' treatment. It is made to look and taste identical to the drug treatment being tested, so that people in the studies do not know if they are getting the placebo or the 'real' treatment. Researchers often talk about the 'placebo effect'. This is where patients feel better after having a placebo treatment because they expect to feel better. Tests may indicate that they actually are better. In the same way, people can also get side effects after having a placebo treatment. Drug treatments can also have a 'placebo effect'. This is why, to get a true picture of how well a drug works, it is important to compare it against a placebo treatment.
A pulmonary embolism can give you chest pain, make you feel breathless and uncomfortable or make you breathe rapidly. A pulmonary embolism is dangerous and can kill you if it is not treated.
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