Extending Anti-Estrogen Therapy: Is More Really Better?

For women with early-stage, hormone receptor–positive breast cancer, anti-estrogen therapy is one of the most powerful ways to reduce the risk of the cancer coming back. We have long known that taking some therapy is better than none — studies show that tamoxifen or aromatase inhibitors (AIs) for five years can cut the risk of recurrence by about 40-50% and lower the chance of dying from breast cancer by about 30% compared with no therapy at all (Davies et al., 2013; Pan et al., 2017). Even women who stop early — after two or three years instead of five — still get some benefit compared to taking no medication, but the protection is not as strong (Davies et al., 2013).

This raises an important question: once those first five years are complete, should you keep going? Many patients wonder whether it’s worth extending treatment to seven, eight, or even ten years — and whether the extra time on medication will meaningfully lower the chance of recurrence without adding too many side effects. A new study helps us answer that question.

A large meta-analysis from the Early Breast Cancer Trialists’ Collaborative Group (EBCTCG), published just a few weeks ago, looked at 12 clinical trials with more than 22,000 women who had already finished two to five years of tamoxifen or AI therapy. The women were randomly assigned to either stop treatment or continue for another three to five years with an AI.

After an average of eight years of follow-up, the study found that continuing anti-estrogen therapy beyond five years led to:

This is one of the clearest answers we’ve had yet that extending therapy — especially for higher-risk women — can make a real difference and the pooling of the studies shed light on the magnitude of benefit.

Extending therapy also means extending side effects. Aromatase inhibitors can cause joint pain, muscle stiffness, hot flashes, and menopausal symptoms that can interfere with day-to-day life. In the EBCTCG's study, about one in three women stopped extended therapy early because of side effects, reminding us that staying on treatment is often challenging (EBCTCG, 2024).

Bone health is also important to consider. Extended AI in this study increased the risk of fractures by about 2%. This may seem small, but fractures — especially hip fractures — can have a big impact on long-term health and independence, particularly for older women (Cummings et al., 2002). The good news is that extended therapy did not increase the risk of stroke, heart attack, or other major health problems.

The decision to continue therapy beyond five years is personal and should be made with your oncology team. Women with higher-risk features — such as lymph node involvement — are likely to benefit the most. Women with lower risk, low bone density, or significant side effects may choose to stop earlier at 5-years.

We often use bone density scans (DEXA scans) to check bone health and weigh the risks and benefits before deciding whether to continue therapy. Shared decision-making is key: what matters most is finding the right balance between reducing your cancer risk and protecting your quality of life.

Treatment for breast cancer continues to evolve. Newer therapies, like CDK4/6 inhibitors, are being studied and used in earlier stages of disease and may reduce the need for long-term AI therapy — or even replace extended AI-benefit for some women. But right now, this new analysis supports that continuing anti-estrogen therapy for up to 10 years can offer extra protection, especially for higher-risk women who can tolerate it well.

Extended anti-estrogen therapy does provide more protection — but it comes with side effects and an increased risk of fractures. For many women, the benefit is worth it. For others, quality of life and bone health may tip the scale toward stopping earlier. The best plan is the one that fits your personal risk, your health, and your life goals. And, as always, your discussion with your oncologist.