The following correction was printed in the Guardian's Corrections and clarifications column, Thursday 27 August 2009

The report below of two studies that assessed the risk of blood clots associated with various brands of contraceptive pill contained errors. We said one study found that the risk of clots varied depending on the type of progesterone and on the dose of oestrogen; we should have referred to progesteron, not progesterone, in this context and throughout the article. We misspelled the names of several pill brands: Trinordial should have Trinordiol; Marvalon should have read Marvelon; Femodin should have been Femodene; Jasmine should have been Yasmin. Trinordiol, which was recommended as being among the safest contraceptives by researchers in one study, has been discontinued in the UK.

Women are not routinely using the safest brand of contraceptive pill, according to two studies aimed at assessing the risk of blood clots associated with the medication.

All types of the combined contraceptive pill carry a risk of venous thrombosis or venal blood clot but some carry a higher risk than others. In one study researchers found that the risk of clots varied depending on the type of progesterone and on the dose of oestrogen.

The safest option, they said, was the combination of hormones found in the widely used pills Ovranette, Microgynon 30 and Trinordial. These, known as second-generation pills, have a low dose of oestrogen combined with levonorgestrel, a type of progesterone.

Women most at risk from venous thrombosis, which includes deep-vein thrombosis in the arms or legs, and pulmonary embolism, are those on third-generation pills such as Marvalon, Mercilon and Femodin, the authors said.

The study also found that Jasmine, a new pill on the market, carried a similar risk of blood clots to Marvalon and Mercilon.

Progesterone-only pills, which have a lower level of progesterone than the combined pill, and hormone-releasing intrauterine devices (IUDs) are not associated with any increased risk of blood clots.

The NHS advises women to use pills with levonorgestrel. But the study, by researchers from the Leiden University medical centre, in the Netherlands, finds that more harmful, third-generation, brands are being widely used.

Women taking the contraceptive pill have a five-fold increased risk of venous thrombosis compared with non-users, the study says. But the researchers stress that the absolute risk of blood clots with use of any type of oral contraceptive in young women is low, affecting less than one in 1,000 users. They recommend, for women of normal weight and no predisposition to blood clots, a low-dose combined pill.

More than 100 million women use the pill worldwide.

The findings were based on a group of 1,524 women aged 18-50 who had suffered a thrombosis. Of the group, which had an average age of 37, 859 had suffered a deep vein thrombosis of the leg, 495 had had a pulmonary embolism and 111 had suffered both. According to the findings, 72% (1,103) women had been on the pill at the time they experienced their thrombosis.

A second study, at Copenhagen University, confirmed the findings of the first. This found that the risk of venous thrombosis decreased with duration of use and declining oestrogen dose.

Lynn Hearton, of the Family Planning Association, said: "Although the combined pill does slightly increase the risk of thrombosis, the risk is still really low."