What Is Cannabis?

Research has been done on the effects of cannabis or cannabinoids on chronic pain, particularly neuropathic pain (pain associated with nerve injury or damage). A 2018 review looked at 47 studies (4,743 participants) of cannabis or cannabinoids for various types of chronic pain other than cancer pain and found evidence of a small benefit. Twenty-nine percent of people taking cannabis/cannabinoids had a 30 percent reduction in their pain whereas 26 percent of those taking a placebo (an inactive substance) did. The difference may be too small to be meaningful to patients. Adverse events (side effects) were more common among people taking cannabis/cannabinoids than those taking placebos. A 2018 review of 16 studies of cannabis-based medicines for neuropathic pain, most of which tested a cannabinoid preparation called nabiximols (brand name Sativex; a mouth spray containing both THC and CBD that is approved in some countries but not in the United States), found low- to moderate-quality evidence that these medicines produced better pain relief than placebos did. However, the data could not be considered reliable because the studies included small numbers of people and may have been biased. People taking cannabis-based medicines were more likely than those taking placebos to drop out of studies because of side effects. A 2015 review of 28 studies (2,454 participants) of cannabinoids in which chronic pain was assessed found the studies generally showed improvements in pain measures in people taking cannabinoids, but these did not reach statistical significance in most of the studies. However, the average number of patients who reported at least a 30 percent reduction in pain was greater with cannabinoids than with placebo.

• There’s evidence from studies in animals that administering THC along with opioids may make it possible to control pain with a smaller dose of opioids.
• A 2017 review looked at studies in people in which cannabinoids were administered along with opioids to treat pain. These studies were designed to determine whether cannabinoids could make it possible to control pain with smaller amounts of opioids. There were 9 studies (750 total participants), of which 3 (642 participants) used a high-quality study design in which participants were randomly assigned to receive cannabinoids or a placebo. The results were inconsistent, and none of the high-quality studies indicated that cannabinoids could lead to decreased opioid use.
• Researchers have looked at statistical data on groups of people to see whether access to cannabis (for example, through “medical marijuana laws”—state laws that allow patients with certain medical conditions to get access to cannabis)—is linked with changes in opioid use or with changes in harm associated with opioids. The findings have been inconsistent.

A small amount of evidence from studies in people suggests that cannabis or cannabinoids might help to reduce anxiety. One study of 24 people with social anxiety disorder found that they had less anxiety in a simulated public speaking test after taking CBD than after taking a placebo. Four studies have suggested that cannabinoids may be helpful for anxiety in people with chronic pain; the study participants did not necessarily have anxiety disorders.

Cannabinoids, primarily CBD, have been studied for the treatment of seizures associated with forms of epilepsy that are difficult to control with other medicines. Epidiolex (oral CBD) has been approved by the FDA for the treatment of seizures associated with two epileptic encephalopathies: Lennox-Gastaut syndrome and Dravet syndrome. (Epileptic encephalopathies are a group of seizure disorders that start in childhood and involve frequent seizures along with severe impairments in cognitive development.) Not enough research has been done on cannabinoids for other, more common forms of epilepsy to allow conclusions to be reached about whether they’re helpful for these conditions.

Glaucoma is a group of diseases that can damage the eye’s optic nerve, leading to vision loss and blindness. Early treatment can often prevent severe loss of vision. Lowering pressure in the eye can slow progression of the disease.

Studies conducted in the 1970s and 1980s showed that cannabis or substances derived from it could lower pressure in the eye, but not as effectively as treatments already in use. One limitation of cannabis-based products is that they only affect pressure in the eye for a short period of time.

A recent animal study showed that CBD, applied directly to the eye, may cause an undesirable increase in pressure in the eye.

• A 2015 review of 23 studies (1,326 participants) on the cannabinoids dronabinol or nabilone for treating nausea and vomiting related to cancer chemotherapy found that they were more helpful than a placebo and similar in effectiveness to other medicines used for this purpose. More people had side effects such as dizziness or sleepiness, though, when taking the cannabinoid medicines.
• The research on dronabinol and nabilone for treating nausea and vomiting related to cancer chemotherapy was done primarily in the 1980s and 1990s and reflects the types of chemotherapy treatments and choices of antinausea medicines available at that time rather than current ones.

• Many studies of cannabis or cannabinoids in people with health problems (such as multiple sclerosis, PTSD, or chronic pain) have looked at effects on sleep. Often, there’s been evidence of better sleep quality, fewer sleep disturbances, or decreased time to fall asleep in people taking cannabis/cannabinoids. However, it’s uncertain whether the cannabis products affected sleep directly or whether people slept better because the symptoms of their illnesses had improved. The effects of cannabis/cannabinoids on sleep problems in people who don’t have other illnesses are uncertain.