A large international clinical trial has found that taking high-dose vitamin D3 alongside standard treatment does not help people with advanced colorectal cancer live longer or keep their cancer under control for longer than a standard daily vitamin D dose.

The findings come from the Phase 3 SOLARIS trial, published in JAMA, and are expected to help oncologists, pharmacists, and other healthcare professionals answer a question many patients ask: can high-dose vitamin D improve cancer treatment?

Researchers enrolled 455 patients with previously untreated metastatic colorectal cancer—a stage where the cancer has spread to other parts of the body. Participants received standard chemotherapy with bevacizumab, a targeted medicine that blocks the growth of blood vessels feeding tumours. They were randomly assigned to receive either high-dose vitamin D3 (8,000 IU daily for two weeks followed by 4,000 IU daily) or the usual 400 IU daily.

The main goal was to see whether high-dose vitamin D could improve progression-free survival, meaning the length of time before the cancer became worse.

It did not.

Patients receiving high-dose vitamin D had a median progression-free survival of 11.8 months, compared with 10.3 months for those taking the standard dose. However, the difference was too small to be considered statistically meaningful, meaning it could simply have happened by chance.

The study also found no improvement in overall survival—the total length of time patients lived after starting treatment. Patients in the high-dose group lived a median of 25.6 months, compared with 27.0 months in the standard-dose group. Likewise, tumour shrinkage rates were similar between the two groups.

The findings are particularly important because an earlier Phase 2 SUNSHINE trial had suggested that high-dose vitamin D might slow cancer progression, raising hopes that the inexpensive supplement could become a useful addition to treatment. However, larger Phase 3 trials are considered the gold standard for testing whether a treatment truly works because they involve more patients and provide stronger evidence.

The good news is that high-dose vitamin D did not cause major additional safety problems. Serious side effects such as low white blood cell counts and high blood pressure occurred at similar rates in both groups. Even so, researchers say there is no evidence that patients gain any cancer-fighting benefit from taking the larger doses.

The results reinforce current recommendations from major cancer organisations that vitamin D should be used to treat deficiency and support bone health—not as an anti-cancer therapy unless future evidence proves otherwise.

For patients, the message is straightforward: while vitamin D remains important for overall health, taking very high doses alongside chemotherapy is unlikely to improve outcomes in metastatic colorectal cancer. Anyone considering supplements during cancer treatment should first discuss them with their oncology team.