Berberine

Berberine is unusual among supplements because it genuinely works — controlled trials show it lowers blood sugar and cholesterol, which is why it got the 'nature's Ozempic' nickname (though it's nothing like a GLP-1 drug, and the effect is modest). But 'it works' cuts both ways: anything with real drug-like activity has real drug-like risks. Its biggest one is interactions — berberine blocks the liver enzymes (CYP3A4 and others) and a transporter that clear a huge range of medicines, so it can push up the levels of statins, blood thinners, heart and transplant drugs, and stack with diabetes medication to cause low blood sugar. It's also contraindicated in pregnancy and newborns. Reasonable for the right person, but not a casual add-on if you take other medications.