Let’s say a jurisdiction outlaws the cultivation, possession, and use of psychoactive salvia. Before the law goes into effect, the police take notes about who seems to have it, who seems to be buying it, and who seems to be growing it. Can the police go to a judge and get warrants to raid those people on the suspicion some of them might still have some now that it’s illegal?
As I understand it, the Constitution disallows ex post facto laws, not warrants issued due to information gathered when the behavior in question was legal: All of the warrants would be issued, all of the raids would be conducted, and all of the arrests would be made after the passage of the relevant laws, and the last time I checked most laws don’t come with grace periods. (Grandfathering is a different concept and I can’t imagine why it would apply here.)
(This might be interesting if there is an ‘internal possession’ charge, such that you are in possession of psychoactive saliva if it is detectable in your urine: You took it before the law passed, but since it’s still inside you after the law passed you’re guilty of possession. See S.D. Codified Laws Sections 22-42-5; South Dakota v. Schroeder, 674 N.W. 2d 827 (2004), upholding conviction for possession of methamphetamine for presence in a urinalysis screening, your local pothead’s paranoid fantasies. New Hampshire has similar law.)