(CNN) Medical marijuana is now legal in 28 states and the District of Columbia. Increasingly, people are turning to cannabis to treat a range of symptoms and conditions, including nausea, bipolar disorder and seizures.
But when it comes to using marijuana to treat chronic pain or post-traumatic stress disorder, two related reviews published Monday in the journal Annals of Internal Medicine find little scientific evidence to support either its effectiveness or its safety.
Up to 80% of people who request medical marijuana want to ease pain, while more than a third cite PTSD as their primary reason, the US Department of Veterans Affairs researchers wrote, citing a separate recent small study.
As a result, physicians increasingly need to talk with patients about the potential benefits and harms of cannabis use, the authors note. Where can doctors turn for evidence-based material?
The researchers found 27 studies that examined the use of cannabis products for chronic pain in adults. But they discovered insufficient evidence to draw conclusions about whether medical marijuana was helpful to pain sufferers, with one exception: neuropathic or nerve pain. Even there, the team found only weak evidence that cannabis alleviates chronic neuropathic pain.
By contrast, they discovered sufficient evidence to conclude that cannabis use among the general population probably increased the risk of car accidents, psychotic symptoms and short-term cognitive impairment.
Next, the team reviewed data from two reviews and three studies assessing plant-based cannabis preparations used to treat PTSD among adults. Here, the authors found the available scientific studies to be flawed due to either small numbers of participants or a failure to include non-cannabis-using control groups.
Once again, the researchers decided the evidence was insufficient and not rigorous enough to draw firm conclusions about the effectiveness of cannabis in this patient population.
Worse still, one study of veterans with PTSD showed a small but "significant" worsening of symptoms among veterans who either started or continued using cannabis during the study period, the team said.
"We found low- to moderate-strength evidence that cannabis use is associated with an increased risk for psychotic symptoms, psychosis, mania, and -- in active users -- short-term cognitive dysfunction," the authors noted.
Dr. Sanjay Gupta, CNN's chief medical correspondent, said "it is still too early to really understand how effective cannabis may be for chronic pain or PTSD or to fully understand the ways in which cannabis could provide relief."
Additional research would take time, said Gupta, who was not involved in the new study but has reported on medical marijuana extensively in three CNN documentaries entitled "Weed." "Weed 3," which first aired in 2015, gave viewers a front row seat to the first federally approved clinical study on the use of marijuana for PTSD.
With respect to the new research, Gupta said that scientists would need to first address safety issues and reliable, consistent dosing strategies.
Cannabis remains a Schedule I substance, the Drug Enforcement Administration classification for drugs believed to have a high potential for abuse along with some potential to create severe dependence. With federal restrictions placed on marijuana, scientists must overcome various legal and procedural hurdles to research it. Gupta noted, "that makes these important studies harder to conduct."
Dr. Dermot P. Maher, an assistant professor in the Johns Hopkins Department of Anesthesiology, noted that the authors searched several sources to identify "under-reported results," which helped create a "more complete picture" of medical marijuana's possible treatment utility.
"The potential benefits and harms of medical marijuana should be evaluated and compared against other available treatments, such as physical therapy, spine interventions, pain medications and surgery," Maher, who did not contribute to the review, wrote in an email. He added that the economic and societal "costs" of chronic pain emphasize "the need to examine all available tools."
In an editorial accompanying the research, Dr. Sachin Patel of Vanderbilt Psychiatric Hospital noted that the findings are not unique. With very little quality evidence available, the efficacy of cannabis -- and cannabinoid products -- for treating pain and PTSD remains unknown.
One possible reason why some people find cannabis helpful in easing pain -- despite scientific evidence suggesting otherwise -- could be that the effects of cannabis depend on potency and dose, Patel wrote in an email. Meanwhile, most studies use lower doses and lower potencies of medical marijuana than what is found in dispensaries.
"Pain is a multifaceted symptom, which includes sensory, cognitive, and emotional components," he wrote. "Perhaps cannabis decreases the clinical effect of chronic pain" in ways unrecorded by pain rating scales. If scientists looked at emotional symptoms, such as anxiety, quality of life and sleep, instead of traditional symptoms, that "may begin to tell us which specific areas of 'pain' are most likely going to be helped by cannabis, if any."
Jane Metrik, an associate professor at Brown University School of Public Health, calls the new research both "timely" and "very well-done." Though she did not contribute to the review, Metrik works as a clinical psychologist at the Providence VA Medical Center and routinely treats veterans with PTSD who use cannabis.
"It's a growing issue that VA providers are grappling with every day," she wrote in an email. Cannabis users often report a "short-term alleviation of PTSD symptoms": Sleep is easier, for example, and tension relieved.
Often, though, they become dependent on the drug, increasing both quantity and frequency, possibly leading to cannabis use disorder, which includes 11 behavioral symptoms outlined in the Diagnostic and Statistical Manual of Mental Disorders-5.
PTSD combined with cannabis use disorder "is frequently associated with greater symptom severity, worse treatment outcomes, and increased use of health services," Metrik wrote. "In light of insufficient evidence on the benefits of cannabis use in patients with PTSD, it is particularly important to increase access to evidence-based trauma-focused treatments for PTSD."
In particular, the VA has highlighted prolonged exposure and cognitive processing therapies, while Metrik recommends cognitive behavioral treatment for insomnia for veterans with problems sleeping.
"Veterans expect cannabis to provide relief from symptoms of combat-related trauma and perceive cannabis to be a low-risk or safe substance unlike other drugs," Metrik wrote. But "the science behind the legislative action of legalizing medical cannabis as treatment for individuals with PTSD is lacking."
She said controlled studies on its effectiveness and safety are needed to substantiate self-reported positive claims and "to test whether benefits of using plant-based cannabis outweigh the risks."