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In July of 2002, U.S. health officials proclaimed that hormone-replacement therapy was not safe. The blockbuster announcement shocked the millions of women who used HRT to treat symptoms of menopause, as well as their care providers.

HRT had, for decades, also been promoted as a cheap, effective way to prevent cardiovascular disease in postmenopausal women. But research showed that, to the contrary, the risk of heart attack and stroke actually increased, as did the risk of breast cancer. HRT also decreased the risk of osteoporosis and colon cancer, but, over all, the risks were said to outweigh the benefits.

The findings generated massive media coverage and led to a sharp drop in prescriptions. Since the announcement, HRT use has fallen by more than half in Canada, to an estimated five million prescriptions in 2007, from about 12 million in 2002.

Five years hence, however, the warnings about the risks of HRT are a lot less categorical and the benefits (as well as the limitations) are more clear.

The current state of knowledge is well laid in the new book by Tara Parker-Pope, an award-winning health columnist for The Wall Street Journal.

HRT: Everything You Need To Know..., while focusing on a single drug treatment, serves as a timely reminder for consumers that health stories (and those about drugs in particular) are rarely cut and dried.

As the HRT story demonstrates, it is dangerous to make assumptions about how a drug does or does not work, it is foolhardy to think that drugs, particularly hormone-based ones, work the same in all age groups and rarely does one study provide definitive answers. Rather, scientific knowledge evolves over time.

First, some background. The Women's Health Initiative was a massive study, launched in 1993, that enrolled almost 162,000 postmenopausal women, aged 50 to 79. Part of the study was intended to examine the risks and benefits of HRT.

In July of 2002, one component of the WHI, which studied the use of estrogen and progestin in women who had a uterus, was stopped early, largely because women taking that HRT regime saw their risk of breast cancer and heart attack increase. In March of 2004, a second component of the WHI, which studied estrogen-only therapy in women who had undergone a hysterectomy (the surgical removal of the uterus), was stopped early, primarily because of an increase in the risk of strokes. (The distinction is that progestin helps to prevent uterine cancer.)

HRT actually works pretty well for the treatment of menopausal symptoms such as hot flashes, vaginal dryness, mood swings, sleep disorders and decreased libido. But worries about the apparent increase in the risk of cancers and cardiovascular disease led many women to eschew treatment.

In her book, Ms. Parker-Pope argues that this was unnecessary. The author says some aspects of the headline-grabbing WHI study were either misleading or plain wrong, that in 2002 scientists made sweeping conclusions based on dubious or partly analyzed data.

Earlier this year, researchers reported - based on the same WHI data - that women in their 50s who took HRT actually had a 30-per-cent-lower risk of dying than those who did not. A second study, also with the same source data, found that women in their 50s taking estrogen alone had a 60-per-cent-lower risk of heart attack.

These 2007 findings seem to be the polar opposite of those in 2002.

But what they tell us is that the age of a woman and the timing of her hormone use dramatically changes risks and benefits. It appears that in the years immediately after menopause HRT lowers cardiovascular risks, but that starting hormones at an older age is risky.

Those important subtleties were lost in the initial findings. Why?

According to Ms. Parker-Pope, researchers did not recruit the right women for the study. Their average age was 63 and the majority had begun HRT 10 years or more after menopause. Of the 27,000 in that component of the study, fewer than 3,500 were under the age of 55.

There has been much reassuring evidence for younger women in recent years. It appears that using HRT to treat the symptoms of menopause for the first three to five years is safe. (But, as Ms. Parker-Pope notes, the "dirty little secret" is that these drugs tend to delay symptoms, but they do not make them disappear.)

Questions about long-term use of hormone-replacement therapy remain unanswered.

What is clear is that reporting on this issue, from the screaming headlines about the horrors of HRT through to the lack of follow-up reporting on the evolving science, has been a disservice to women.

There are enough bothersome symptoms of menopause already; we don't need to add fear and confusion to the list.