Hormone replacement therapy (HRT) is a medical treatment regime many women going through menopause choose to take as a way to ease symptoms. Analysis of 52 separately performed studies has shown that even short-term use of hormone therapies significantly increased risk of ovarian cancer. The pooled studies include over 21,000 women from North America, Europe, and Australia. The increased risk can be as high as 40 percent depending on conditions surrounding the patient.

The research came out of the University of Oxford, and was published in The Lancet. Lead researcher Sir Richard Peto explained “the claim that there is no risk for a couple years worth of HRT simply isn’t true.” The Mayo clinic predicted the discovery will likely change general treatment guidelines for the millions of women every year who, suffering from hot flashes and other symptoms, turn to hormone therapies.

Hormone replacement for menopause is meant to supply hormones that the body no longer makes after menopause. Women who used HRT for the average length of time, two to five years, were more likely to develop one of the two most common types of ovarian cancer compared to women who had never participated in hormone therapy, according to researchers from the International Collaborative Group on Epidemiological Studies of Ovarian Cancer. Those types are serous epithelial and endometrioid ovarian cancer, but HRT was not linked to an increased risk of mucinous or clear cell ovarian cancers, the other twp types of ovarian cancer. The specific types of HRT involved in the study are preparations containing oestrogen only or oestrogen together with progestogen, two types of hormones lost during menopause.

Previous studies have pointed to the dangers of using hormone replacement therapies. A large clinical study suggested that the estrogen-progestin pill Prerempo posed more risks to patients than benefits. With this news doctors became less willing to prescribe HRT. Those risks included the development of breast cancer, or heart disease, stroke, and blood clots. The Lancet study is the largest ever study to assess the link between HRT and ovarian cancer, and provide a quantified risk.

The increased risk for ovarian cancer was not changed by the age at which HRT began, body size, past use of oral contraceptives, hysterectomy, alcohol use, tobacco use, or family history of breast and ovarian cancer.