BACKGROUND: Tormentil extracts (TE) have antioxidative properties and are used as a complementary therapy for chronic inflammatory bowel disease. In individual patients with ulcerative colitis (UC) positive effects have been observed. GOALS: To assess the safety, pharmacology, and clinical effects of different doses of TE in patients with active UC. STUDY: Sixteen patients with active UC [clinical activity index (CAI) ≥5] received TE in escalating doses of 1200, 1800, 2400 and 3000 mg/d for 3 weeks each. Each treatment phase was followed by a 4-week washout phase. The outcome parameters were side effects, CAI, C-reactive protein, and tannin levels in patient sera. RESULTS: Mild upper abdominal discomfort was experienced by 6 patients (38%), but did not require discontinuation of the medication. During therapy with 2400 mg TE per day, median CAI and C-reactive protein improved from 8 (6 to 10.75) and 8 (3 to 17.75) mg/L at baseline to 4.5 (1.75 to 6) and 3 (3 to 6) mg/L, respectively. During therapy, the CAI decreased in all patients, whereas it increased during the washout phase. Neither undegraded nor metabolized tannins could be detected by liquid-mass spectrometry (LC-MS) in patient sera. CONCLUSIONS: TE appeared safe up to 3000 mg/d. Tannins from TE are not systemically absorbed. The efficacy in patients with UC should be further evaluated.