Background
Phase III, double-blind, placebo-controlled RCT. 7154 women aged 35–70 at elevated breast cancer risk (family history, prior LCIS/atypical hyperplasia, or other risk factors) randomized to tamoxifen vs placebo for 5 years. IBIS-I is one of four major tamoxifen prevention trials (alongside NSABP P-1/BCPT, the Italian trial, and Royal Marsden) and the longest-running, with 20-year follow-up available.
Interventions and follow up
Arm A: Tamoxifen 20mg daily for 5 years
Arm B: Placebo for 5 years
Primary endpoint: Breast cancer incidence (invasive + non-invasive)
mFollow up: 16 years (long-term follow-up)
Arm B: Placebo for 5 years
Primary endpoint: Breast cancer incidence (invasive + non-invasive)
mFollow up: 16 years (long-term follow-up)
Results
BC incidence (all, 16-yr): RR 0.71, 95% CI 0.60–0.83, P<.0001 (~29% reduction)
ER+ invasive BC: RR 0.63, P<.0001
ER– BC: RR 1.01 — no benefit
BC mortality: Not significantly different
ER+ invasive BC: RR 0.63, P<.0001
ER– BC: RR 1.01 — no benefit
BC mortality: Not significantly different
Adverse events
Thromboembolic: Venous thromboembolic events RR 2.35 (tamoxifen vs placebo).
Gynecologic: Endometrial cancer RR 2.54 (~5 additional cases/1000 women); vaginal discharge increased.
Vasomotor: Hot flashes substantially increased. No increase in non-gynecologic cancer. Benefit/risk favors younger women (<50) with lower absolute VTE and uterine cancer risk.
Gynecologic: Endometrial cancer RR 2.54 (~5 additional cases/1000 women); vaginal discharge increased.
Vasomotor: Hot flashes substantially increased. No increase in non-gynecologic cancer. Benefit/risk favors younger women (<50) with lower absolute VTE and uterine cancer risk.
Conclusions
Tamoxifen reduced breast cancer incidence by approximately 29% at 16-year follow-up in high-risk women. The benefit persists beyond the 5-year treatment period (carryover effect). Benefit is restricted to ER+ BC; ER– cancers are not prevented. No mortality benefit was demonstrated.
Key Limitations
No BC mortality benefit despite robust incidence reduction — chemoprevention's ultimate goal (survival) remains unproven. Uterine cancer and VTE risks require risk stratification (contraindicated with prior VTE or uterine cancer). In postmenopausal women, aromatase inhibitors (IBIS-II, MAP.3) have supplanted tamoxifen as first-line chemoprevention due to more favorable side effect profiles. Adherence over 5 years is a major practical barrier.
Clinical Context
IBIS-I confirmed the NSABP P-1 finding of tamoxifen's preventive efficacy. Tamoxifen remains the primary chemoprevention option for premenopausal high-risk women (anastrozole/exemestane are postmenopausal only). ASCO recommends discussing chemoprevention with women at ≥1.7% 5-year Tyrer-Cuzick or Gail model risk. Uptake in clinical practice is <10% despite guideline endorsement. ESMO-MCBS: not applicable (prevention trial).