The distinctive tang of Earl Grey tea is provided not by the type of tea itself but by a flavoring extracted from the peel of a bergamot orange (Citrus bergamia or Citrus aurantium bergamia), grown mainly in and around Calabria, Italy. According to Wikipedia: "Bergamot grow on small evergreen trees known as bergamots, which are produced from a cross of the pear lemon and the Seville orange or grapefruit."
Medicine Dispatch reports on one possible side effect from over-indulgence in Earl Grey:
When consumed in excess, the bergamot oil in Earl Grey tea can induce muscle cramps, fasciculations [twitches], paraesthesias [numbness] and blurred vision.
These symptoms, according to People's Pharmacy, are thought to be caused by bergapten,
a natural compound that can block the flow of potassium in and out of cells. Muscle cells rely on potassium flow, so that might explain the connection. Most people do not seem to suffer this kind of reaction, so those who do may be exceptionally sensitive.
Another component of bergamot oil is bergamottin:
a natural furanocoumarin [an organic chemical compound] found principally in grapefruit juice. It is also found in the oil of bergamot, from which it was first isolated and from which its name is derived. . . . Along with the chemically related compound 6’,7’-dihydroxybergamottin, it is believed to be responsible for the grapefruit juice effect in which the consumption of the juice affects the metabolism of a variety of pharmaceutical drugs.
Now, whether the bergamot oil contained in a bergamot orange or in Earl Grey tea triggers the effect has not been studied, as far as I can tell. According to the Grapefruit Juice Drug Interaction website,
'Seville oranges' contain 6',7'-dihydroxybergamottin, and did seem to cause a similar drug interaction to GJ, but these are not used for commercial juice production and are only seen in the marketplace as the main oranges used in marmalade production. Evidence now exists for pummelos (pomelos) to cause similar drug interactions to grapefruit. More exotic grapefruit hybrids such as tangelos, have not been directly studied, and since they are not commonly seen in the marketplace, may never be studied. However, these products are genetically derived from grapefruit originally, and therefore may have potential for drug interactions.
On the other hand, Cholesterol Network reports that "Although other citrus fruits such as limes and Seville oranges (sour oranges) do contain some similar chemical compounds to grapefruits, there have been no significant reports of them leading to harm with statins."
There is also some reason to believe that the metabolism of cyclosporin (an immunosuppressant taken by transplant patients) is adversely affected by Earl Grey tea.
Recent research suggests that a traditional English breakfast with Earl Grey tea and toast with bitter orange marmalade should markedly modify cyclosporin metabolism. The flavonoid naringenin and the furanocumarin bergamottin are strong inhibitors of one subfamily of CYC P450 enzymes in grapefruit juice and also occur in bitter oranges (Citrus aurantium), as well as in their variant bergamot, which flavours Earl Grey tea. In animal experiments, bitter oranges induced severe cyclosporin intoxication at an otherwise safe cyclosporin dosage. (Nowak & Nowak)