Berberine is a chemical extracted from goldenseal (Hydrastis canadensis),barberry (Berberis vulgaris), and Oregon grape (Berberis aquifolium), which has antibacterial activity. However, one double-blind study found that 100 mg berberine given with tetracycline (a drug closely related to doxycycline) reduced the efficacy of tetracycline in people with cholera. In that trial, berberine may have decreased tetracycline absorption. Another double-blind trial found that berberine neither improved nor interfered with tetracycline effectiveness in cholera patients. Therefore, it remains unclear whether a significant interaction between berberine-containing herbs and doxycycline and related drugs exists.