Women who experience hot flashes, night sweats and painful joints when taking the anti-cancer drugs tamoxifen and Arimidex may breathe easy: these are signs that show the drugs are working.

According to a new study led by Jack Cuzick of Cancer Research UK, the chances that their breast cancer would return were lower for women who received either anastrozole or tamoxifen to prevent the breast cancer from returning and developed vasomotor symptoms, than for those who were treated with these drugs, but didn’t experience side-effects. Also, they were more likely to be cancer-free for a long term.

Using data from 4,000 women with early-stage breast cancer, the researchers found that for women who developed joint symptoms after the treatment, the recurrence rate was reduced by about 41 percent, while for those who developed more vasomotor symptoms, the recurrence rate decreased by 47 percent.

“At the moment all we can say is that the symptoms indicate the likely success of the treatment,” Professor Jack Cuzick said, adding that these symptoms, usually called menopausal symptoms, show that there are lower levels of estrogen in the body. Both drugs decrease the effects of estrogen in post-menopausal women with breast cancer. Anastrozole decreases the amount of estrogen that circulates in the body. Previous studies have shown that high estrogen levels are directly linked with a higher risk of breast cancer recurrence.

The study was published in the journal Lancet Oncology.

The researchers hope that the knowledge that the presence of these vasomotor symptoms indicates the positive results of the treatment will encourage women to keep taking the drugs long-term.