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★☆☆Contested — the evidence disagrees with itself, or the finding carries a caution. A caution or conflict always shows one star, however strong the sources are.
Oregano's reputed blood-sugar drop comes from rats fed an oil blend
Obese rats fed oil blends that included oregano had lower blood sugar on the best blend. The warning about oregano and diabetes drugs rests on that, not on a human study.
What the study found
The blood-sugar drop tied to oregano came from rats fed a mix of oils
Rats bred to be obese and insulin-resistant, fed blends of fenugreek, cinnamon, cumin, oregano and other essential oils, had lower blood sugar on the best blend, and lower blood pressure. The advice to watch oregano with diabetes drugs traces back to mixtures like these, in rats.
The caveat · No test with diabetes drugs yet
Oregano was one oil among several, so its own share cannot be told apart. The study was a pilot comparing three blends. No study of oregano taken with metformin or another diabetes drug has reported results.