Drug Interactions

Analysis of dilutions of extract of E. angustifolia and E. purpurea showed medium to high levels of inhibition of cytochrome P450 3A4 in vitro. Testing of chicoric acid and echinacoside alone showed low to very low inhibition.

The authors did not speculate on what other compounds may be present in either Echinacea sp. that caused the more significant enzyme inhibition (35).

Caffeine, tolbutamide, dextromethorphan, and oral and intravenous midazolam were given to 12 healthy subjects. This was followed with a course of E. purpurea, 400 mg four times a day for 8 days, then these drugs were given again and the subjects were assessed for cytochrome P450 activity. They found that the Echinacea significantly reduced the activity of hepatic CYP 1A2 and intestinal CYP 3A and induced hepatic CYP 3A activity. They advised caution when giving drugs that are metabolized by these same enzyme systems (36).

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