Who advised you, at your age, to double investment risk (and reward) with margin? I've never borrowed to invest. I know no respected investing text that recommends the frequent use of margin except in rare cases.
Buffett DOES use leverage, but it's at about 1.6 ratio and it comes, almost free, from his own insurance holdings. Thus, BRK has little to fear from margin calls. Buffett has said that he doesn't advise the average investor to use margin.
Certainly using margin to buy a small non-diversified foreign medical startup like RCEL is generally dangerous.
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