While millions of ordinary American citizens in 20 states and the nation's Capitol can legally purchase medical marijuana at the nearest dispensary and those living in Washington state and Colorado can purchase the drug recreationally, scientists and researchers find themselves out of the loop.

According to the Washington Post, psychiatrist Sue Sisley, who studies post-traumatic stress disorder, has had to wait four months so far to obtain samples of the Control I substance from the government.

The reason for the long delays, which can slow down researchers' work by months or even years, is because the U.S. government essentially has a monopoly on growing and cultivating marijuana used for scientific research.

The process to get the tightly controlled drug requires three separate levels of approval, according to the Post.

The amount of medical marijuana users in the nation are estimated to be more than 1 billion, the Post reported.

Despite its wide use and vast approval from citizens and physicians, the government has still placed a negative stigma around marijuana, which has many scientists frustrated that federal authorities haven't streamlined the process yet.

For researchers who have been able to study the drug's effects, there is a strong indication that marijuana can treat cancer, HIV and Alzheimer's disease.

Throughout the years, the Drug Enforcement Administration has declined numerous petitions to reclassify marijuana from a Schedule I drug, which is the designation for the most dangerous drugs without having any medical uses.

To obtain marijuana for scientific studies, researchers must apply to the Department of Health and Human Services to buy research-grade samples from the U.S.'s only weed farm at the University of Mississippi. The National Institute on Drug Abuse manages the weed farm, which is the only one allowed under federal law.

If approved by HHS, the researcher must seek the permission of the DEA to possess and transport the marijuana samples.

DEA spokesperson Dawn Dearden said the agency supports research for medical marijuana but under the Controlled Substances Act, there has to be regulations to follow.

"DEA has not denied DEA registration to a HHS-approved marijuana study in the last 20-plus years," Dearden said.

Sisley, who was approved by the HHS this month, said the approval is a "triumph" for marijuana research but is still doubtful she can complete her work anytime soon because of the DEA's infamous delays.

"There is a desperate need for this research," Sisley said, adding, "but it's impossible to study this drug properly in an atmosphere of prohibition."