Rue.

Rutin was first isolated from rue and named after it, and the plant remains a recognised source material. A rue preparation therefore already contributes rutin, and adding isolated rutin adds to the same pool. The whole plant brings furanocoumarins and alkaloids that the isolated flavonoid does not. That difference is the entire reason isolated rutin exists as a separate ingredient.

The rutin in rue is quercetin with a disaccharide attached, and gut bacteria have to cleave that sugar before the quercetin is absorbed. So the plant contributes indirectly to the same aglycone pool a quercetin supplement supplies directly. How much arrives depends on the individual's gut flora. Counting the two separately overstates total intake.

Rue carries furanocoumarins that absorb UVA, and St John's Wort carries hypericin, which is also photosensitising at sufficient intake. Combining them stacks two independent routes to the same skin reaction under sun exposure. The two also pull opposite ways on CYP3A4, since St John's Wort induces the enzyme while furanocoumarins inhibit it, making the net effect on any co-administered drug unpredictable. This is a combination to avoid rather than to design around.

Both ingredients raise the absorbed fraction of co-administered compounds by suppressing intestinal first-pass metabolism, through overlapping targets. Stacking them compounds an effect that is already the reason each is added. For anything with a narrow margin between a useful and an excessive dose, that is a problem rather than a feature. The interaction extends to any prescription medication in the same category.

Berberine affects CYP enzymes and efflux transporters, and rue furanocoumarins inhibit CYP3A4 by a mechanism-based route. Together they push the same first-pass system in the same direction. The practical consequence is unpredictable exposure to anything else being taken. Read it as a caution about a shared metabolic bottleneck.

Silymarin inhibits several drug-metabolising enzymes in laboratory systems, though its clinical significance at ordinary intakes has been debated. Combining it with a strong mechanism-based CYP3A4 inhibitor moves the combined effect further from baseline. Both plants are commonly stacked in the same formulas, which is why the overlap is worth naming. The evidence for silymarin here is in vitro, and that should be stated.

Ascorbate and rutin have been co-formulated since the 1930s, which is where the historical label vitamin P came from. That label was withdrawn because the flavonoids do not meet the definition of a vitamin. The pairing persists as convention and for antioxidant stability in the formulation. It is not evidence that either does more when combined.

Flavonol glycosides bind non-heme iron in the gut and reduce absorption from the same meal. Rue is flavonol-rich, so the same chemistry applies. The magnitude will be smaller than with a strongly tannin-bearing herb. Spacing a mineral supplement away from a botanical dose is the general answer.

No pharmacological interaction between rue and melatonin is established, and none is claimed here. The relevant point is that photosensitisation from furanocoumarins depends entirely on UVA exposure, so anything that shifts the timing of outdoor activity changes the risk profile. This row is included to keep the exposure dependency visible. Regard it as context rather than a mechanism.