Can we do drug tests on employees because our client requests it?

Q. We are a California security guard company currently working under contract for a public utility. Our contract states that we must drug test guards and make available the results if requested by our client. I know we can test applicants before they are employed, but we need to test current employees as well to be compliant with our contract. Many weren't tested before being hired. Can we test current guards who were not tested before being employed because our client requests it? Can I give a copy of a drug test of one of my employed guards if my client requests it?  Lisa, California