The incidence and severity of interactions of herbal products with calcineurin inhibitor (CNI) metabolism in renal transplant recipients have not been systematically investigated. These patients have a high rate of herbal product consumption, including products interfering with CNI metabolism. The study aimed at identifying an impact of herbs and foods on CNI metabolism in a cohort of renal transplant recipients by conducting dietary interviews (1) in patients with very low and high CNI maintenance dose requirements and (2) by retrospective analysis of unexplained marked deviations from CNI baseline trough levels. Of 73 renal transplant recipients, 59 were treated with a CNI-based immunosuppressive regimen. Seven patients with an exceptionally high or low CNI dose were interviewed. Five of these seven patients had not consumed any plant product with known influence on CNI metabolism. In one patient chicory-coffee and bitter chocolate had been suspected as contributing to high CNI dose requirement, but the dose could not be lowered after discontinuation of these foods. Participating nephrologists reported three as yet unexplained temporary deviations from baseline CNI trough levels, of which two could be linked to newly started consumption of high volumes of herbal teas and the other to St. John's wort. Consumption of herbal products within the study cohort had no detectable impact on maintenance doses of CNI. However, herbal products, and specifically teas when consumed by the liter, could be linked to temporary strong deviations from CNI trough levels. The study demonstrates that as yet unnoticed herbal interactions with CNI can be detected by detailed dietary analysis, but that the overall impact on maintenance doses of CNI appears to be low.