SAN FRANCISCO — Law firms and marijuana have long intersected in criminal court.

But as pot goes mainstream, firms are getting into the business side of the multi-billion-dollar industry.

Over the past several years, small and mid-sized firms have started marijuana divisions that advise pot-related businesses about issues such as where they can locate, what their websites and product labels can say and how to vet new clients.

Experts say the work is not without its perils. While numerous states have legalized medical marijuana and some have even approved recreational use, the drug remains illegal under federal law.

Attorneys say they focus on providing advice about what the law does and does not allow and decline to answer questions about how clients can bend the rules.