Brewing an Interaction: Non-commercial Green Tea-Associated Supratherapeutic Tacrolimus Levels Presenting With Gastrointestinal Symptoms.

Tacrolimus is a calcineurin inhibitor with a narrow therapeutic index and can have significant interactions with foods and herbs that affect CYP3A4 and P-gp metabolism. We report a case of gastrointestinal symptoms associated with supratherapeutic tacrolimus levels due to heavy consumption of non-commercial green tea with orange extract in a 44-year-old woman with Primary Sclerosing Cholangitis status post liver transplant and refractory Crohn's disease. She presented with worsening abdominal pain, nausea, and bloating while on a stable tacrolimus dose with previously therapeutic trough levels. Evaluation showed a supratherapeutic tacrolimus trough of 18.3 ng/mL without evidence of infection, hepatic dysfunction, or acute inflammatory bowel disease flare. Endoscopic findings were unchanged from prior examinations. Tacrolimus was held and green tea discontinued, leading to rapid normalization of tacrolimus levels and complete symptom resolution within 24 hours without corticosteroid escalation. Tacrolimus was later resumed at the prior dose with stable therapeutic levels during follow-up. This case highlights the potential for non-commercial herbal preparations, including green tea products, to alter tacrolimus metabolism and cause clinically significant toxicity.