You can't do human trials for an experimental drug if there's an already-approved drug that would potentially work (which is sensible - you don't want the patient to die if the experimental drug turns out to be worse than what was already available). This is actually one of the main things slowing cancer research, as the only people participating in trials have already gone through multiple rounds of treatment without success, and are so full of metastatised tumours and are so unwell that they're likely to drop dead any day even if given a miracle treatment. It's hard to get a new treatment approved if everyone who had it died afterwards, even if using it as a first-line treatment would probably save millions of lives.

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