PHOENIX - Arizonans who get a doctor's recommendation to buy medical marijuana are going to have to shell out a little extra money for the state - and maybe a whole lot more.

Attorney General Tom Horne said Wednesday that the marijuana sold at state-regulated dispensaries being set up under Proposition 203 is subject to the state and local sales taxes.

Horne figures the levy could generate $40 million for the state, based on a Denver Post story on how much marijuana is sold through dispensaries in that community "and applying that pro-rata to the Arizona population."

With that in mind, Rep. Steve Farley, D-Tucson, wants to go further. Much further. With some Republican support, he is proposing a 300 percent tax on marijuana sold at dispensaries.

Farley had no estimate on how much his proposal would bring in. But using Horne's $40 million as a starting point, a 300 percent levy could produce $1.8 billion a year - far more than enough to erase the state's projected $1.1 billion deficit for next year.

Horne acknowledged that the law, approved by voters, specifies only those who have a recommendation from a doctor are entitled to buy the drug, and that prescription sales are tax-exempt.

"But that statute didn't use the word 'prescriptions,' " Horne said. "It used the word 'written certification.' "

That was deliberate.

A 1996 voter-approved law allowing doctors to prescribe marijuana was never implemented because the U.S. Drug Enforcement Agency threatened to revoke all prescription-writing privileges of any physician who wrote such an order for a substance that remains illegal under federal law. This new law, mirroring the successful practice in other states, avoids that term.

Horne said as far as he's concerned, that ends the discussion.

"Since they're not prescriptions, then in my view it's taxable, like anything else is taxable," he said.

Horne's finding means the Arizona Department of Revenue will instruct dispensaries to collect not only the 6.6 percent state levy, but any local sales taxes.

Andrew Myers, who managed the pro-203 campaign, expressed some concern.

"We're not wild about the idea of increasing the cost of what essentially is medication for seriously ill people," he said, but no challenge is planned.

But Myers said what Farley wants would be challenged as illegal.

He said it's one thing to tax marijuana like other products. A special tax, Myers said, runs afoul of a constitutional provision barring lawmakers from altering voter-approved measures.

Farley, however, said the tax is justified. He said a 300 percent levy puts the tax on marijuana at the same general level as the tax on cigarettes, which are subject to a $2-per-pack levy.

"People use cigarettes as an over-the-counter medication for various types of things," he said. He also doubts imposing the tax alters what voters approved.

Anyway, Farley said, those who really need the marijuana won't mind paying the extra fee. He figures marijuana sells for $40 an ounce, meaning the sales price, tax and all, would be $160.

Myers said $40 marijuana is a "myth," and the actual price at dispensaries will be 10 times that, putting medical marijuana out of reach of many in need, particularly since the drug is not covered by health insurance.