Just two months after Minnesota launched its medical marijuana program, the high costs are beginning to hound some patients, driving them back to the black market to afford treatment.

Five patients point to how the "highly restricted and regulated system" is costing them hundreds to thousands of dollars a month, with none of that cost covered by insurance. They have reverted to buying marijuana illegally.

State data show that about one in five of the 491 registered patients had not returned to buy medication in August, although state officials point to different possible factors behind this.

In Minnesota, a small vial can cost nearly $130, more than double the price of a similar product in Colorado, where recreational marijuana is legal.

The companies hired to make marijuana products—only pills and oils, no leaf products—defended their prices, saying costs will fall over time. They have also set up charitable organizations to buy down patients' medication and offer discounts to low-income segments.

There is no shortage of success stories in the Minnesota program so far, from reports of parents who have witnessed their children's seizures lessen to stories of patients who have been cured of chronic pain.

The return to buying from the illegal pot trade, however, leads to broader challenges.

Patrick McClellan, a sufferer of muscular dystrophy and symptoms such as muscle spasms, said that his monthly expense for legal state-sanctioned vaporizer pens can cost as much as $264, when he said he could get a month's worth of marijuana buds for $80 on the street.

"[It] is an expensive designer drug that only the rich can afford right now," McClellan said.

Jonathan Holmgren, faced with about $2,000 in medical marijuana bill, returned to using the raw plant—still deemed illegal by the state—to treat his Crohn's disease. Two other state-registered patients also reverted to buying marijuana on the street but requested anonymity out of fear of arrest.

Part of the price difference with other states' medical marijuana offerings is competition since only two companies are allowed in Minnesota. Oils and pills cost more to make and the customer base is small, with hardly any sign of the state legislature reversing the prohibition on raw leaf any time soon.