The universe of marijuana laws may be devolving into chaos, but Bill McPike remains a mellow man. For 30 years, the Fresno, Calif., attorney has defended clients against pot charges and counseled entrepreneurs on setting up shop to legally grow and sell the substance that spawned a thousand slang terms.

But whatever its name over the years — Mary Jane, weed, wacky tobaccy — at no other point has the law been so unsettled. A collision of federal policy, state statutes, local ordinances and old-fashioned politics means more clients for McPike to defend and more uncertainty to navigate. Although U.S. Attorney General Eric Holder Jr. said last year that federal prosecutors would back off cases involving medical marijuana usage, authorities in California and elsewhere recently have vowed to rein in what they see as an unwieldy industry of pot growers and consumers.