Nothing in science can be "proven". Proof only exists in mathematics and logic. You can't prove something is safe, but you can provide evidence that it is unsaf

Nothing in science can be "proven". Proof only exists in mathematics and logic. You can't prove something is safe, but you can provide evidence that it is unsafe. And generally the best evidence for something being unsafe is unfortunately only available after something is in widespread use because then the links to cancer, birth defects, or whatever can be found. There are lesser forms of evidence based on testing on animals and cell lines than can be done before the widespread use, but these have a lot of both false positives and false negatives.