You may well have heard this point made before, and I surely have myself, but it nevertheless made me grin, again, today:
We can only ask supporters of the precautionary principle to follow it through to its logical conclusion, that is not to have it applied unless it can be proved that no risk is involved. It is up to them to prove that this principle is harmless.
Those are the concluding words of Precaution with the Precautionary Principle, published (pdf only but in both French and English) by the Institut Economique Molinari. My thanks to Cécile Philippe of the IEM for the email that pointed me to this publication, and to this conclusion.