A US study has revealed that 36% of postmenopausal breast cancer patients stop taking medication early because side effects are more severe than they expect.

Researchers at Northwestern University in Chicago, US, said that over a third of the 686 women involved in the study, who are being treated for oestrogen-sensitive breast cancer, stopped using drugs that help to prevent the disease from reoccurring because of joint pain, hot flushes, nausea and weight gain.

As part of the study, patients filled out a 46-question survey rating their quality of life, and the symptoms associated with breast cancer and treatment.

After three months, a 33-35% of women had severe joint pain, 28% had hot flushes, 24% had decreased libido and 14-17% experienced anxiety.

These numbers increased the longer the women were treated.

The women at highest risk for quitting the medications before the recommended five years are those who still are experiencing residual side effects from recent chemotherapy or radiation therapy, according to the study.

As a result of the side effects, 10% of the women had stopped taking the drug within two years. A further 26% had stopped by four years.

Lead investigator Lynne Wagner said she was concerned that clinicians consistently underestimate the side effects associated with treatment because patients are dishonest about their symptoms.

"Patients don’t want to be complainers and don’t want their doctor to discontinue treatment. So no one knew how bad it really was for patients," Wagner said.

The drugs, aromatase inhibitors, stop the production of oestrogen in postmenopausal women, whose breast cancer cells are stimulated by oestrogen.

About two-thirds of breast cancers are oestrogen sensitive, and aromatase inhibitors reduce the recurrence of cancer in postmenopausal women.