Canadian researchers have found that the popular anti-depressant Paxil may interfere with breast cancer treatment — causing more patients to have relapses or even die, in the worst cases.

Women who took Paxil while also taking tamoxifen — the standard therapy for breast cancer — at the same time were more likely to die of their breast cancer, and the longer both drugs are used together, the higher the risk becomes.

Researchers recommend speaking to a doctor immediately to switch to another antidepressant as the study did not find any other medications to increase risk.