Obese women on the pill are 30 times more at risk of a fatal brain clot, a new study has warned.
Cerebral venous thrombosis (CVT) is a rare type of stroke and oral contraceptives puts women with a body mass index above 30 at greater risk than normal weight women not taking it.
It affects around three in every million people in the UK and most are young women.
A new Dutch study was the first to investigate whether obesity and CVT was linked.
Although the risk was low, scientists said women should be informed of the risk and offered alternative contraceptives that are not linked to thrombosis.
Risk factors for CVT overlap some with those for venous thromboembolism (VTE) and include cancer and oral contraceptives but there also are risk factors specific to CVT including local infections and head trauma.
The study involved 186 men and women with CVT from two hospitals and 6,134 healthy controls for comparison.
CVT patients had an average age of 40 compared to 48 in the control group, there were more women, more often used oral contraceptives and more frequently had a history of cancer compared with control participants.
Obesity defined as a BMI above 30 was associated with increased risk of CVT and that the association appeared due to the increased risk among women taking oral contraceptives.
There was a nearly 30-fold increased risk of CVT among obese women taking oral contraceptives compared with women of normal weight not taking oral contraceptives.
There also was an increased risk of CVT in overweight women who used oral contraceptives.
However, there was no association between obesity and CVT among men or women who did not use oral contraceptives.
Dr Jonathan Coutinho of the Academic Medical Centre, Amsterdam, said: "The increased risk of VTE and CVT associated with oral contraceptives in the presence of obesity might make physicians reluctant to prescribe oral contraceptives to obese women.
"However, although the relative risks are increased substantially, the absolute risks of CVT are small.
"Moreover, withholding oral contraceptives may lead to an increase in unintended pregnancies and thus the number of pregnancy-related thrombosis cases.
"Nevertheless, obese women should be informed about the increased risk of thrombosis if they use oral contraceptives, especially if other risk factors are present.
"Alternative methods of contraception that are not associated with thrombosis, such as intrauterine device, might be offered to these women."
The study was published online by JAMA Neurology.
In an editorial Dr Chirantan Banerjee of the Medical University of South Carolina added: "The authors correctly point out that despite the manifold increased relative risk, the absolute risk of CVT in obese women taking oral contraceptives still remains low and should not preclude oral contraceptives use among them.
"Use of oral contraceptives has also been associated with increased risk of arterial ischemic stroke in obese women.
"Better counselling and education of obese women informing them of the increased risk would be prudent, as would be consideration of alternate nonhormonal oral contraceptives options."