Berberine is an alkaloid found in barberry and several other plants, and it has become one of the most popular supplement ingredients of the last decade.
Memorial Sloan Kettering's About Herbs monograph reports: “Repeated oral intake of berberine at a dose used in some studies significantly decreased 2D6, 2C9, and 3A4 activities.”
Those three cytochrome P450 enzymes metabolise a very large share of ordinary prescription medicines. Slowing them down can raise the level of a drug in the body, which is why the monograph warns directly: if you are taking CYP2D6, 2C9 or 3A4 substrate drugs, berberine may decrease their effectiveness.
It carries a second, separate caution that belongs on this page: berberine “may worsen jaundice in infants or result in kernicterus, a condition in which prolonged high bilirubin levels can cause irreversible effects”. That is the specific reason we say plainly that this is not a product for anyone pregnant or nursing.
The same monograph is candid about benefit: most studies are of limited quality, and in the cholesterol literature many were heterogeneous, of low quality or at high risk of bias.
And it states no dose of its own. It refers only to “a dose used in some studies”, which is the honest position when the literature does not converge — and which means there is no figure to compare an unpublished amount against.