A phase II trial gave grape seed extract to 41 men whose prostate cancer had returned after local treatment, shown by a rising PSA. PSA doubling time lengthened, but the trial had no placebo group. This is an early human signal, not proof of benefit.
Could something as simple as grape seed extract actually slow cancer growth? The clinical evidence is surprisingly strong for grape seed.
In one colorectal cancer study, mice carrying human tumors were given grape seed extract oil. After 8 weeks, their tumors were 44% smaller compared to controls. In another 2025 mice study, researchers gave grape seed extract and vitamin C to mice with colon cancer. The reduction was 76.6% versus 68.8% on doxorubicin chemotherapy.
In animal models of colorectal cancer, prostate cancer, breast cancer, and other kinds of cancers, grape seed extract inhibited cell growth and induced apoptosis, which is cancer cell death.
But mice aren't people, so here's what really got my attention. A phase two human trial was just published involving 41 men whose prostate cancer had returned after local treatment evidenced by a rise in PSA, but no other evidence of metastasis. They received 150 mg of a grape seed extract twice daily, and PSA doubling time increased from 5.7 months to 6.9 months. Even more interesting, 78% of the men experienced at least some increase in PSA doubling time. Moreover, the treatment was generally well tolerated.
But there's an important limitation to this study. There was no placebo group, and PSA doubling time is a surrogate marker. We don't yet know whether grape seed extract reduces metastasis or improves survival. However, the laboratory evidence is intriguing, and now we have a human signal.
Here's the evidence. You decide.
Educational research summary, not medical advice. Talk with your oncologist and pharmacist before starting, stopping or changing any treatment or supplement.