Breast cancer patients who report more side effects of their hormone therapy drugs and less confidence communicating with their doctors are more likely to intentionally or unintentionally miss a dose, according to a new study. 

“Endocrine” therapy, given as a daily tablet that acts to prevent hormones from helping the tumor to grow, is an important part of treatment for some types of early breast cancer. But many women do not stick to the medications over time, the authors write in The Breast.

“We were surprised that so many women admitted to nonadherent medication-taking behaviors,” said lead author Gretchen Kimmick of Duke Cancer Institute at Duke University Medical Center in Durham, North Carolina.

“There are many reasons that women may choose not to take the medicine,” she told Reuters Health by email. “The most common reason is probably because they notice side effects.”

The researchers studied 112 postmenopausal women with early stage, hormone-receptor positive breast cancer who had completed surgery, chemotherapy or radiation and were currently taking an endocrine therapy medication like tamoxifen.

Endocrine therapies for breast cancer can cause side effects like nausea, hot flashes and headaches.

The women answered survey questions about their levels of general pain, fatigue, hot flashes or night sweats, and nerve pain over the previous week. They also rated their confidence in their ability to take the medicine as prescribed, their ability to communicate with their doctors, their beliefs about hormone therapy, and their medication behavior.

Women rated from “never” to “very often” the frequency with which they forgot to take medication, skipped doses, or reduced or stopped medication when feeling better or when feeling worse.

On average, the women had been receiving endocrine therapy for about three years. More than half said they had missed some doses unintentionally, due to having a busy day, being away from home, or problems getting refills.

A third said they had intentionally skipped or changed the dose, often because they felt worse or had side effects, and sometimes because of cost.

Women with less confidence in taking their medications and less confidence talking with their doctors were more like to miss doses, the researchers found.

Colin McCowan, professor of health informatics at the University of Glasgow, who wasn’t involved in the study, told Reuters Health by email that women might not usually receive advice, support and monitoring for adherence during endocrine therapy.

Failing to take these drugs as directed increases the risk of cancer recurrence, death, lower quality of life and also higher health service costs, said McCowan, who was not part of the new study.

In the U.K., about 70 percent of postmenopausal women with breast cancer will be prescribed endocrine therapy, which blocks cancer cells from growing again after chemotherapy or surgery, he said. Only around half of women complete their five-year course of endocrine therapy, he said.

The side effects can cause “huge disruption to women's lives,” and women who don’t understand the benefits of the medicines may be less motivated to take them, McCowan said.

Having regular conversations with your doctors about taking endocrine therapy and any side effects, and trying to incorporate the pills into your daily routine, may help patients stick to the medication, he said.