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There is some evidence that large does of plant sterols can cause muscle aches in susceptible people. You didn't mention what kind of grapefruit juice/beverage you drank 2-4 cans of daily. Pure grapefruit juice is known to interfere with metabolism of many medications, either increasing or decreasing their intended effect. If your beverage is only grapefruit-flavored (like Fresca) then this couldn't possibly be contributory. The complete answer here is that no studies have been done with sterols and grapefruit juice, so any interaction is only anecdotal.
Scuyler, MSN, RN, ANP
If it is from the plant sterols then the problem is the same as with statins. There is a warning that statin-associated myopathy is associated with concomitant use of grapefruit juice. In all cases of statin-associated myopathy, the muscle pain went away within a few days of stopping the statins. At this point I would want to consult with my doctor and have some blood tests run to see exactly what is causing the problem.
Schuyler, MSN, RN, ANP