TORONTO — Doctors and clinics are receiving fees of as much as $350 per patient from medical-marijuana producers eager to snag customer referrals, despite professional rules in some provinces that explicitly outlaw such payments, producers say.

Physicians working under federal law do not prescribe a specific brand of cannabis, but some try to steer patients toward particular cannabis suppliers – and expect reimbursement, industry members report.

Doctors sometimes tell companies the money is to subsidize the cost of verifying patient information, or providing marijuana-related services to patients not covered by medicare.

But one government-approved pot grower charges the payments are essentially “kickbacks.” Professional ethics rules in most of Canada prohibit physicians’ receiving “rebates, fees, gifts or other incentives” in return for patient referrals.

Nanaimo, B.C.-based Tilray showed the National Post copies of unsolicited invoices it has received from two individual doctors, three clinics and two “patient aggregators” that work with...