The course is going to begin with a discussion of the classic English blend, Earl Grey, a combination of black tea with bergamot, so I have been looking into the chemistry of bergamot in more detail.
As you know, bergamot is a member of the citrus family, and its oil contains a host of chemicals that are common to many different kinds of citrus, grapefruit in particular. In fact an important chemical, bergamottin, gets its name from the bergamot, but is more abundant in grapefruit. It is one of a class of compounds called furanocoumarins.
The reason that bergamottin is important is that—together with 6',7' dihydrobergamottin, another chemical in bergamot and grapefruit—it inhibits an essential enzyme family involved in drug metabolism, called cytochrome P450.
The consequences of this inhibition are neither simple and nor predictable. For example, some medications are detoxified by cytochrome P450 enzyme in the gut and possibly the liver, so inhibition of these enzymes by bergamottin and similar compounds will increase the blood levels of the drug you get from a given dose. For some drugs the result is toxicity, which may be serious and even possibly life-threatening. However, for other drugs, inhibition of cytochrome P450 may mean that you could get the desired effect at a lower, less toxic dose.
|Bergamot —Image from Wikipedia.|
Look up "grapefruit juice effect" in Wikipedia to get an idea of what drugs are involved, as well as more information about the bergamottin/drug interactions.
Incidentally, there are other common additives to teas that may have the same effects, including angelica in particular.*
While I have not found reports associating reasonable amounts of Earl Grey tea with drug metabolism problems, you should be aware that these problems may exist, and not just with Earl Grey, but with other blended teas and herbal teas.**
And always, consult your doctor about drug/diet interactions!
* Guo LQ, Yamazoe Y. Inhibition of cytochrome P450 by furanocoumarins in grapefruit juice and herbal medicines. Acta Pharmacol Sin. 2004 Feb;25(2):129-36.
** Rainer Nowack, Barbara Nowak. Herbal teas interfere with cyclosporin levels in renal transplant patients. Nephrology Dialysis Transplantation, Volume 20, Issue 11, 1 November 2005, Pages 2554–2556.