Does a Marijuana prescription protect you from a positive drug test by your employer in a medical M state?

Does a marijuana prescription for a non-life threatening illness protect you from a positive drug test by your employer in a medical marijuana state? Is it a state by state issue? I would love to get a California lawyer's view on this.

: http://alcoholdrugstreatment.info/does-a-marijuana-prescription-protect-you-from-a-positive-drug-test-by-your-employer-in-a-medical-m-state