Women with diabetes and primary HER2-positive and hormone receptor–positive breast cancer fared better when they used metformin vs insulin in a randomized trial involving nearly 8,400 individuals.

Investigators conducted a substudy of the ALTTO trial, assessing diabetes patients treated with or without metformin. They looked at disease-free, distant disease-free, and overall survival vs patients without diabetes. Among the results:

• 95% had no diabetes history; ~2% had diabetes without metformin treatment, and the remainder had diabetes treated with metformin.
• At a median follow-up of 4.5 years, HR+ patients with diabetes not treated with metformin experienced worse disease-free, distant disease-free, and overall survival.
• Metformin’s effect helped women with HER2+ and HR+ disease; insulin’s impact on patients with these subtypes was unfavorable.