What happened, in summary

A 57-year-old woman had multiple invasive ductal cancers in her left breast together with high-grade ductal carcinoma in situ. Her tumor was estrogen-receptor positive and progesterone-receptor positive. In May 2022, she began paclitaxel with trastuzumab. By late July, the documented treatment had transitioned to trastuzumab alone.

During treatment, she asked a pharmacy service to review a self-selected supplement containing St. John’s wort and lemon balm that she hoped would reduce chemotherapy-related stress. The review identified a potentially important interaction between St. John’s wort and paclitaxel because the herb can increase CYP3A4 activity and potentially lower exposure to the chemotherapy drug.

She had not yet started the supplement, so the clinical team advised against taking it. This avoided a potential reduction in paclitaxel activity. Her cancer treatment continued with trastuzumab after the documented paclitaxel portion of therapy. The case emphasizes the importance of checking herbal and dietary products before using them alongside systemic cancer treatment.