His concern is hoarding for an unproven use. This could deny patients who it’s proven need the drugs - for lupus for example - access to them.
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Okay, this actually would be a somewhat sensible reason. Has he stated this anywhere?
There will be ample supplies of the drug, and doctors don't withhold treatment from critically ill people in order to preserve medical supplies for the less ill except in extreme circumstances.
Actually, its not a sensible reason because #1 most people who have chronic illnesses like lupus and other autoimmune diseases have their own supply and #2 if youre really concerned about hoarding then ban the hoarding, like the President is doing, not the administration of a potentially life saving drug.