Would be interesting to find some research in this area: most of what I can find warns of increased side effects from maintaining constant dosage.
Interesting related info: "Seville oranges, (often used in marmalades), limes and pomelos also produce this interaction. Varieties of sweet orange, such as navel or valencia, do not contain furanocoumarins and do not produce this interaction."
I don't know why, but this could be why.
Could grapefruit juice protect against diabetes?
https://www.nhs.uk/news/diabetes/could-grapefruit-juice-prot...
Most generic drugs probably don't cost more than a few cents a pill to manufacture, assuming the pharmacy and manufacturer make a profit on it.
In other words, it's all in the R&D and compliance costs, which grapefruit couldn't help with.
And re: doctors prescribing a combination of active ingredients that hadn't had a pharma company spend a few billion dollars studying... well, just read this[2], about how some company is making big bucks selling $300 fish oil pill bottles that had a bit of FDA compliance work (and related studies) done about them because doctors would never prescribe the $30 pill bottles that weren't filed with the FDA.
[1] http://bvssp.icict.fiocruz.br/pdf/Pharmaceutical.pdf
https://slatestarcodex.com/2014/06/15/fish-now-by-prescripti...