Maybe someone can post a list of supplements that SHOULD NOT be taken with grapefruit?
I don't think you'll find a lengthy, definitive list of dietary supplements that interact dangerously with grapefruit. You can find some excellent lists of medications that interact, though, by simply asking google.
The general rule to grapefruit is that it will interact with substances that have a low bioavailability and are metabolized by the CYP 3A4 enzyme, so you might be able to research a particular supplement to see if there is any harmful possibilities going from those two aspects.
Otherwise, off the top of my head, I can only think of a few supplements that can be harmful with grapefruit.
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Red Yeast Rice - or anything containing statins which are meant to lower cholesterol and reduce risk of cardiovascular disease. The interaction with RYR increases the risk of myopathy.
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Designer Steroids - if you think of them as "supplements" they may interact badly with grapefruit in the case that they happen to be methylated, with significant androgenic activity. An increase in bioavailability/half-life from grapefruit here would put more stress/damage on the liver and possibly other organs.
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Caffeine - there might be some unpleasant CNS overstimulation if you all of a sudden start taking significant doses of caffeine in a short period of time. This would be like 300-400mg or more taken at once, depending on personal tolerances. So, really one can take smaller doses of caffeine when using grapefruit juice to get the desired effects. The lethal dose of caffeine is quite high, so I don't think we need to worry about it here. With grapefruit the LD.50 might be 50 cups of coffee instead of 100 for the average adult male.
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Considering the site where I'm posting, I guess I should post some non-supplemental things that could interact to give people some idea.
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SERMs may interact. Nolvadex is listed as possibly interacting, but the significance is unknown right now. I don't believe it's significant because Nolvadex has a high bioavailability to begin with, but other might want to think more about their choice to use grapefruit with a SERM.
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Cabergoline - it's a dopamine agonist, but might be used by some to try to control prolactin induced gynecomastia. Grapefruit can cause increased blood levels of cabergoline, and trust me, you don't want to mess around with overstimulating your dopamine receptors if you like mental stability.
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The Benzos - the breakdown of these will be slowed by grapefruit, and could potentially build to toxic levels.
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There's probably many other choices that are unhealthy, but few that are truly harmful, dangerous or fatal. I'm sure it would be easier to find information if it were the case that people were dying or being hospitalized for using grapefruit with dietary supplements.