The amino acid taurine is marketed as a nutritional supplement. Cao et al. now show that taurine can enhance the malignant behaviour of cancer cells, but also increase the survival and effector functions of CD8+ T cells. In mouse models of gastric cancer and melanoma, intratumoural injection of taurine promoted tumour growth in immunocompromised mice, but inhibited tumour growth in immunocompetent mice. Further analysis showed that tumour cells consume taurine via the membrane transporter SLC6A6, and that SLC6A6 expression is a negative prognostic indicator for different types of cancer. The authors found that tumour cells can outcompete CD8+ T cells for taurine by overexpressing SLC6A6, which induces endoplasmic reticulum (ER) stress in the CD8+ T cells. This promotes expression of the transcription factor ATF4, which upregulates immune checkpoint genes and induces T cell exhaustion. Taurine supplementation in mice repressed Atf4 transcription and promoted the survival and function of CD8+ T cells. In mouse models of gastric cancer, chemotherapy upregulated SLC6A6 in cancer cells. However, taurine supplementation increased the efficacy of chemotherapy, indicating that its capacity to re-invigorate CD8+ T cells outweighs its direct tumour-promoting effects. A clinical trial has been initiated to test oral taurine in combination with chemotherapy or immunotherapy in patients with gastric cancer.