What are the risks and side effects of cannabis?

Authors: SCIRE Community Team | Reviewer: Andrea Townson and Janice Eng | Published: 2 April 2019 | Updated: Apr 20, 2020

The safety of medical cannabis use after SCI is not yet known. However, a number of risks and side effects of cannabis use in the general population are known. Many of the short-term side effects of cannabis have been reported to be mild to moderately severe and related to the dose of the drug taken. Uncommon but serious adverse effects may also exist. Furthermore, the risks to long-term users are not well known and some side effects may be related to regular use over time.

This is not a complete list. Speak to your health provider for detailed information about the risks and side effects of cannabis use.

Short-term side effects of cannabis may include:

Diagram of the human body showing the different side effects cannabis can have on the body.

Cannabis can cause many side effects to different body systems.13

  • Dizziness and lightheadedness
  • Dry mouth, throat irritation, and cough
  • Drowsiness
  • Altered judgment and attention
  • Anxiety and agitation
  • Hallucinations
  • Disorientation and confusion
  • Increased heart rate
  • Impaired coordination and balance
  • Impaired short-term memory
  • Headache
  • Paranoia and psychosis
  • Reddening of the eyes
  • Decreased intra-ocular pressure (pressure within the eyes)
  • Muscle relaxation
  • Interactions with other medications

Because cannabis lingers in the body long after use, task performance may be impaired for up to 24 hours. It is recommended to avoid operating heavy machinery or performing dangerous activities for 3-4 hours after inhaling cannabis, 6 hours after oral ingestion of cannabis, and 8 hours if a “high” is experienced. Examples of high-risk activities may include performing transfers and participating in physical therapy sessions.

Long-term cannabis use may be associated with:

  • Addiction and withdrawal
  • Airway problems like chronic bronchitis
  • Possible increased risk of mental disorders like anxiety, depression, schizophrenia, and psychosis in people at risk for these conditions
  • Possible increased cancer risk with long term smoking, although this is not yet clear

An emerging concern is the effects that cannabis use may have on adolescents and young adults. Studies have suggested that cannabis use early in adolescence may alter brain development and could be related to the development of psychotic disorders as adults.

Overdosage of cannabinoids

Overdoses of cannabis, although not common, have been reported. The risk increases when both oral and inhaled forms of cannabinoids (prescription or recreational) are combined. The signs and symptoms of overdose are generally tolerable and overlap with the effects of THC such as dizziness, drowsiness, and sensory impairment. More severe complications including psychosis and convulsions occur rarely.

Unlike cannabis, synthetic cannabinoids carry a greater risk of overdose because they are more potent than THC. The clinical presentation of toxicity will depend on the specific synthetic cannabinoid used, but can be severe and even result in death. Since its introduction into the United States in 2008, there have been cases of adverse reactions in all 50 states. There is currently no antidote to synthetic cannabinoids, making the illegal use of these drugs an emerging public health threat. If you or someone you know experiences an overdose, seek medical attention immediately.