Dr. Sanjay Gupta, CNN's chief medical correspondent, has opposed the use of medical marijuana continuously until he published an editorial Thursday stating he had changed his opinion.

Gupta has spent the past year working on a documentary titled "Weed," to be aired on CNN at eight p.m. ET Aug. 11. In the documentary, he said he traveled the world to speak upfront and straightforward with doctors, experts, growers and patients about marijuana.

In his editorial, published Thursday on CNN.com, he said the experience changed his opinion entirely.

"I apologize because I didn't look hard enough, until now. I didn't look far enough," he wrote. "I didn't review papers from smaller labs in other countries doing some remarkable research, and I was too dismissive of the loud chorus of legitimate patients whose symptoms improved on cannabis."

Cannabis has been a schedule 1 substance for 43 years, ever since Aug. 14, 1970, when the Assistant Secretary of Health Roger O. Egeberg wrote a letter recommending it. Sanjay included an excerpt from the letter in his piece.

"Since there is still a considerable void in our knowledge of the plant and effects of the active drug contained in it, our recommendation is that marijuana be retained within schedule 1 at least until the completion of certain studies now underway to resolve the issue," read the letter.

Gupta noted earlier research that Egeberg claimed was not available to him. In 1944, New York Mayor Fiorello LaGuardia commissioned research into the risks of marijuana. It was found to be not as addictive as cocaine or heroin and three times less than tobacco.

The research concluded that nine to ten percent of adults who tried marijuana would become addicted. Cocaine and heroine, both schedule 2 drugs, were found to be addictive at 20 and 25 percent rates respectively. With tobacco, 30 percent of adults who try it become addicted.

"There is clear evidence that in some people marijuana use can lead to withdrawal symptoms, including insomnia, anxiety and nausea. Even considering this, it is hard to make a case that it has a high potential for abuse," Gupta wrote. "The physical symptoms of marijuana addiction are nothing like those of the other drugs I've mentioned. I have seen the withdrawal from alcohol, and it can be life threatening."

The CNN health expert also said he found research papers written from 1840-1930 about medicinal marijuana's benefits, mostly. A search through the U.S. National Library of Medicine also gave him 20,000 more recent articles. Gupta said, by his calculations, six percent of existing studies in the U.S. aimed to find the benefit of medical marijuana, while the rest aimed to prove its harmfulness.

Gupta noted the many ailments marijuana effectively treats, like neuropathic pain, pain from breast cancer and symptoms of post-traumatic stress disorder. All medical issues, he said, are complicated to understand and hard to treat. They also are currently treated with drugs like oxycodone and morphine, which can be ineffective as pain relievers.

"When marijuana became a schedule 1 substance, there was a request to fill a 'void in our knowledge,'" Gupta wrote. "I promise to do my part to help, genuinely and honestly, fill the remaining void in our knowledge."