US Edition However, the drug still dramatically lowered the chances that women would be diagnosed with a second breast cancer.
Scientists say that women should not stop taking the drug, commonly prescribed for five years after a woman has had breast cancer, but that doctors should make them aware of the risk.
Women who did develop a second breast cancer while on the drug were four times more likely than normal to have the more aggressive form of the disease, which is harder to treat.
Tamoxifen works by blocking oestrogen, which can help to fuel breast tumours.
The study compared results from cancer patients taking the drug with patients who had had breast cancer and were not given Tamoxifen.
The findings show that those who received the drug were 60 per cent less likely to develop an oestrogen receptor-positive, or ER-positive, second breast cancer, the most common form of the disease.
However, they were 440 per cent more likely to develop the least common form of the cancer, ER-negative.
"This is of concern, given the poorer prognosis of ER-negative tumours, which are also more difficult to treat," said Dr Christopher Li, from the Fred Hutchinson Cancer Research Centre, in Seattle.
While the study showed that there was a link between using the drugs long-term and an increased risk of developing an ER-negative second cancer, the findings do not suggest that breast cancer survivors should stop taking hormone therapy to prevent a second cancer, Dr Li insisted.
He said: "It is clear that oestrogen-blocking drugs like Tamoxifen have important clinical benefits and have led to major improvements in breast cancer survival rates. However, these therapies have risks, and an increased risk of ER negative second cancer may be one of them.
“Still, the benefits of this therapy are well established and doctors should continue to recommend hormonal therapy for breast cancer patients who can benefit from it."
Dr Sarah Cant, from Breakthrough Breast Cancer, said: “This research confirms the benefits of taking tamoxifen, which has played a major part in improving survival rates for breast cancer.
“Although it suggests women who take tamoxifen for five years or more may be at a slightly increased risk of a specific type of breast cancer in the other breast, the overall risk of this happening is still low.
“Women with breast cancer should be aware of the risks of taking any course of treatment. Breakthrough advises that anyone who is concerned should not stop their medication and should speak to their doctor.”
The study looked at 1,103 breast cancer survivors.
In total 367 of the women went on to develop a second breast cancer.
Of these six times as many were ER-positive as ER-negative.
Tamoxifen has been widely used to treat the disease for more than 20 years.
Each year 45,000 women are diagnosed with breast cancer, a third of whom will go on to die from the disease.