In July 2002 the Women's Health Initiative halted one arm of its hormone therapy trial, and the coverage that followed cut hormone therapy prescribing by more than half within a year. Two decades of reanalysis have substantially revised that interpretation - but the original headlines still shape how many women, and many clinicians, think about HRT.

What the trial actually studied

The WHI enrolled women with an average age of 63, more than a decade past menopause on average, and used two specific formulations: conjugated equine estrogens and medroxyprogesterone acetate, a synthetic progestin. It was designed to test whether hormone therapy prevented chronic disease in older women, not whether it safely relieved symptoms in women in their early fifties. Most women seeking HRT today do not resemble the trial population, and most are not offered the trial's formulations.

What it found

The combined estrogen-progestin arm showed a small absolute increase in breast cancer, stroke, and venous thromboembolism, alongside reductions in fracture and colorectal cancer. The estrogen-only arm - women who had had a hysterectomy - showed no increase in breast cancer, and long-term follow-up has suggested a possible reduction. That distinction was largely absent from the original coverage.

The timing hypothesis

Subsequent age-stratified analyses found that women who started hormone therapy under 60, or within ten years of their final period, had a materially different risk profile from those who started later - including a neutral-to-favourable effect on cardiovascular outcomes. This "timing hypothesis" now underpins the guidance from the Menopause Society and equivalent bodies internationally.

Where guidance stands now

For healthy symptomatic women under 60 and within ten years of menopause, the benefits of hormone therapy generally outweigh the risks. Transdermal estrogen avoids the clot signal associated with oral routes. Micronized progesterone appears to carry a more favourable profile than the synthetic progestin used in the trial. The arbitrary five-year stopping rule that followed 2002 is no longer recommended; duration is a decision to revisit annually rather than a deadline.

What has not changed

Hormone therapy is not appropriate for everyone. Active or recent breast cancer, unexplained vaginal bleeding, active liver disease, and a history of estrogen-dependent cancer or unprovoked clotting all require individual clinical evaluation. Risk is personal, and absolute risk - not relative risk - is the number worth discussing with your clinician. Our menopause HRT guide covers how the current evidence is applied in practice.

Understanding relative and absolute risk

A headline that says a treatment raises a risk by 25 percent sounds alarming until you learn the starting point. In the WHI combined-therapy arm, the extra breast cancer cases amounted to about one additional case per thousand women per year of use. That is a real risk, and it deserves honest discussion, but it is far smaller than the relative figure implies. When you weigh any treatment, ask for the absolute numbers: how many more or fewer women out of a thousand are affected, and over what period of time.

Not all hormone therapy is the same

The trial used oral conjugated equine estrogens with a synthetic progestin. Today, most women are offered transdermal estradiol, and many are given micronized progesterone instead of a synthetic progestin. Observational data suggest that these choices have a more favourable profile for clots and possibly for breast cancer, although large randomised trials of these specific combinations are lacking. Route, dose, and formulation all matter, and they are part of the reason current guidance is individualised instead of one-size-fits-all.

Who should still avoid hormone therapy

The reinterpretation of the WHI does not mean that hormones are safe for everyone. A history of breast cancer, hormone-sensitive cancers, blood clots, stroke, or active liver disease generally rules out systemic therapy, and women with unexplained vaginal bleeding need that investigated first. If you are over 60 or more than ten years past menopause, the balance shifts, and a careful discussion with your clinician is essential. Local vaginal estrogen is a notable exception, since very little is absorbed, and it is usable by many women who cannot take systemic therapy.

How to have the conversation with your clinician

Bring your family history, a list of your medications, and your own priorities. Ask what the benefits would be for your particular symptoms, what the absolute risks are for someone with your profile, and how often the decision will be reviewed. A good clinician will welcome those questions. Our article on common HRT myths and our menopausal hormone therapy guide are useful companions to read before your appointment.

Frequently asked questions

Did the WHI study prove HRT causes cancer?

No. It found a small increase in breast cancer with combined estrogen and a synthetic progestin in older women, and no increase with estrogen alone. The findings have been refined considerably since.

Is it safe to start HRT at 50?

For most healthy women, starting within ten years of menopause and before 60 has a favourable balance of benefits and risks, but the decision should be individual.