California Code, Health and Safety Code - HSC § 11710

(a) An action by an individual user of an illegal controlled substance is governed by the principles of comparative responsibility. Comparative responsibility attributed to the plaintiff does not bar recovery but diminishes the award of compensatory damages proportionally, according to the measure of responsibility attributed to the plaintiff.

(b) The burden of proving the comparative responsibility of the plaintiff is on the defendant, which shall be shown by clear and convincing evidence.

(c) Comparative responsibility shall not be attributed to a plaintiff who is not an individual user of a controlled substance, unless that plaintiff willfully and knowingly gave the individual user money for the purchase of the illegal controlled substance.