More than a dozen people, many of them medical marijuana patients or providers, testified Monday against a measure to tax medical marijuana dispensaries, an effort to head off any black market when sale of state-taxed recreational marijuana starts at the end of this year.

The bill, which had a public hearing before the House Finance Committee, would hit dispensaries with a tax equal to 25 percent of their sales of cannabis and cannabis-infused products.

The bill sponsors have said they’re trying to avoid a dual market — one taxed, one not — as the state moves toward creating a regulated system for the fledgling marijuana industry created by Initiative 502.