Aromatase inhibitors are the medication given to postmenopausal breast cancer patients to reduce the level of estrogen in those whose tumors were fueled by the hormone.
About two-thirds of breast cancers are estrogen-sensitive, and aromatase inhibitors have been shown to reduce the risk of cancer recurring.
About a third of breast cancer patients stop taking medication because side-effects are more severe than they expect, US researchers suggest.
The Northwestern University team questioned 686 women who were taking aromatase inhibitors as treatment for estrogen-sensitive breast cancer.
It found 36 percent stopped their medication because of symptoms such as joint pain, hot flushes, weight gain and nausea.
Patients in the Northwestern University study filled out a 46-question survey rating their quality of life and symptoms associated with breast cancer and treatment.
They were asked about their symptoms before treatment and at three, six, 12 and 24 months after starting treatment.
After three months, a third of women had severe joint pain, 28 percent had hot flushes and 24 percent had decreased libido among a range of symptoms.
The longer women were being treated, the more reported side-effects.
Those at highest risk of stopping before the recommended five years were those still experiencing side-effects from chemotherapy or radiotherapy.
As a result of the side effects, 10 percent of the women had stopped taking the drug within two years. A further 26 percent had stopped by four years.
The researchers say there is a big gap between what women tell their doctors about side-effects and what they actually experience.
Dr. Lynne Wagner led the study, which is being presented to the Annual San Antonio Breast Cancer Symposium.
Dr. Wagner said, “This is a wake-up call to physicians that says if your patient is feeling really beaten up by treatment, the risk of her quitting early is high.”
Dr. Susie Jennings, senior policy officer at Breakthrough Breast Cancer, said: “It is worrying if breast cancer patients are stopping lifesaving treatment early without consulting their doctor.
“The patient information leaflets do warn women of all the side effects, but it is important there is a continued discussion throughout treatment between patient and doctor.”