More than 10% of women with breast cancer stop taking aromatase inhibitors prescribed for their condition because of joint and muscle pain, research has shown.
A US trial showed 13% of women stopped taking the drug, which blocks the production of oestrogen.
Findings were based on a study of 100 women, who were initially enrolled on a trial to study how genetics play a role in the way individuals metabolise drugs and experience side effects.
They were assigned to take one of two aromatase inhibitors, exemestane or letrozole and were followed for six months.
Side effects, if reported, included joint and muscle pain and manifested themselves within two months of treatment starting.
Aromatase inhibitors are most commonly used after surgery, chemotherapy or radiation therapy to prevent cancer from returning.
Study author N Lynn Henry, lecturer in internal medicine at University of Michigan Comprehensive Cancer Center, said: 'Tamoxifen has been around 20-30 years and has a long track record. We know its benefits and risks. Aromatase inhibitors are new, and we don't have as much experience with them. We have to see in the long term which one ends up being better.'
Ms Henry will present the study findings at the 2007 Breast Cancer Symposium in San Francisco.