Short answer: Large studies suggest a small rise. A pooled analysis of 52 studies found that current users had about 1.4 times the risk of never-users, even with under 5 years of use, which its authors put at about one extra ovarian cancer per 1,000 women using hormones for 5 years from around age 50. In the WHI estrogen-plus-progestin trial, the rise (4 versus 3 cases per 10,000 women-years) was not statistically significant.
The Premarin, Prempro, Climara and EstroGel labels each have an ovarian cancer paragraph under malignant neoplasms (section 5.2). They report two findings:
The labels add that the exact length of use linked to a higher risk is unknown.
The Collaborative Group on Epidemiological Studies of Ovarian Cancer pooled data on individual women from 52 studies (Beral 2015). In the prospective studies, 12,110 women developed ovarian cancer, and 55% of them had used hormone therapy.
| Group | Relative risk |
|---|---|
| Current users with under 5 years of use | 1.43 |
| Current or recent users, any duration | 1.37 |
| Serous tumors (most common type) | 1.53 |
| Endometrioid tumors | 1.42 |
| About 10 years after stopping long-term use (serous or endometrioid) | 1.25 |
The risk was similar in European and American studies and for estrogen-only and combined products. It was clearly raised only for the two most common tumor types, and it fell the longer ago use had stopped.
A relative risk of about 1.4 sounds large, but ovarian cancer is uncommon, so the absolute change is small. The authors estimated that if the link is causal, 5 years of use from about age 50 leads to about one extra ovarian cancer per 1,000 users and about one extra death per 1,700 users. See relative vs absolute risk.
These are observational studies, which can be affected by differences between women who choose hormones and women who do not; the authors wrote that the increase may well be largely or wholly causal. The WHI trial, the only randomized data in the labels, was too small to settle it.
Personal and family history of cancer, the reason for treatment and how long hormones are planned are part of weighing this risk. The labels say to use the lowest effective dose for the shortest time that fits the goals and to review the plan regularly. Dose Track can record the product, dose and start date, so the total time on hormones is easy to see.
Read more on Dose Track: Relative vs absolute risk · Meta-analysis · Prempro side effects · Menopause