The precautionary principle has its merits, but I think it's often cargo-culted and its limitations not well articulated. In cases such as GM crops where the stakes may within a human lifetime be "devestating famine" versus "less famine", or cases like vaping where although the harm is unknown it's certainly less than what it's replacing (vaping is uncontroversially less harmful than smoking) the precautionary principle actually has a harmful effect in itself.