The news last November he'd been terminated was devastating for Casias, 29, who took great pride in his job, once earning the honor of Associate of the Year.
"It hurts. It hurts because I care. I care a lot about the store. I always wanted to make sure I do well," he told ABC News.
Casias started taking the medicine last June to cope with pain from sinus cancer and a brain tumor. He says the rare form of cancer causes him pain constantly and he almost died when he was first diagnosed.
Casias sprained his knee at work last November and underwent the routine drug test that follows all workplace injuries. Questioned about his positive test, Casias told management about his condition and presented a state card authorizing his marijuana use for medical purposes, but he was fired anyway. Casias says management told him Walmart does not honor medical marijuana cards.
"I just can't believe that it has to be this way. I don't see why they have to fire me," he said.
Michigan is one of 14 states where medical marijuana is legal, but employers in the Wolverine state can and do terminate employees who fail drug tests. Walmart policy, like the policy of other companies, indicates that in states such as Michigan which allow marijuana use for medical purposes, the store can still terminate an employee following a positive drug test. The law in Michigan says employers do not have to accommodate the ingestion of marijuana in the workplace or employees working while under the influence.
Casias has been receiving Michigan unemployment compensation since his termination and was notified that the company would challenge his eligibility for benefits during a phone hearing on March 31. A Walmart spokesman says that's not the case -- it won't challenge Casias' eligibility.
Even still, Casias thinks he may be entitled to compensation because he did not break any Michigan laws by using marijuana for pain. "I'm currently taking a look at all my options available," he said. "I am currently talking to an attorney at this point."
"This is just an unfortunate situation all around," Walmart spokesman Greg Rossiter told ABC News. "We're sympathetic to Mr. Casias' condition, but like other companies we have to consider the overall safety of our customers and our associates, including Mr. Casias when making a difficult decision like this."
Certainly using a mind-altering drug on the job is something that would trouble any employer, but Casias says he never used the marijuana before or during work. He was in severe pain regularly so it would make sense that the drug was detected given that it can take weeks for marijuana to leave the system.
Doctors say a number of medicines or combination of medicines, such as antihistamines, could pose a danger at the workplace. And medical marijuana, which is thought to be relatively safe, falls into the category of substances that could be hazardous to the workplace. Casias unloaded trucks for Walmart.