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“Compared with pretreatment values, the clearance of caffeine was reduced by about 54% and 55% after one treatment cycle with gestodene- and the levonorgestrel-containing oral contraceptive , respectively.”
European Journal of Clinical Pharmacology • 2004 | View Paper
“Intake of levonorgestrel (0.125 mg) daily for 14 days reduced the metabolism of caffeine slightly, but the elimination of metamizol-metabolites is not influenced.”