Safety and administration

Safety and administration

Medicinal cannabis is currently an unapproved medicine in Australia that requires Therapeutic Goods Administration (TGA) approval for a doctor to prescribe it to a patient. As such, it is important both doctors and patients understand the safety requirements of using medicinal cannabis, as well as the different ways it can be administered.
medicinal cannabis safety ratio chart

Acute toxicity of medicinal cannabis

Medicinal cannabis products are regarded as having low acute toxicity. However, concurrent use of other drugs may mask the effects of cannabis and severe toxicity. Thus, adverse cardiovascular effects and death may be under-recognised.

A report from the World Health Organisation (WHO) stated a lethal dose of THC for a 70kg human would be around 4 grams, and that such a dose would not realistically be possible from either oral consumption or vaporising. In another report, the WHO said that cannabidiol (CBD) had a good safety profile, and that it had relatively low toxicity, though noted that not all the potential effects of CBD had been explored in research to date.

Safety ratio of medicinal cannabis

The concept of a “safety ratio” refers to the difference between an effective dose of a substance and a lethal dose of that same substance. The greater the ratio between these two numbers, the wider the gap between the amount of the drug needed for intended effect and the amount that would likely result in death.

The safety ratio of cannabis (when smoked) is 1000; this means a person would have to take in 1000 times as much as an effective dose before a lethal overdose was reached. By comparison, the safety ratios of drugs like codeine (20) and morphine (15) are much smaller, meaning a lethal dose of these drugs can result from as little as 15 times as much as the effective dose.

It should be noted that these figures are based on research modelling a median quantity of the given substance used by an average 70kg adult. Significant differences exist with respect to a person’s physiological and psychological reactions.

(For more information, see Gable, Robert S. Comparison of acute lethal toxicity of commonly abused psychoactive substances. Addiction. 99. 686–696 (2004))

Possible adverse effects and drug interactions

Possible adverse effects and drug interactions

While cannabinoids are less likely to be associated with severe adverse events than, for example, opiate medicines, details of adverse events with most cannabinoids are lacking.

However, a recent randomised controlled trial of cannabidiol (CBD) for drug-resistant seizures in Dravet syndrome showed significant adverse events including sedation, convulsions and gastrointestinal disturbance in some patients. CBD and THC, being lipid plant extracts, require metabolism before excretion.

The safety ratio of cannabis (when smoked) is 1000. A person would have to take in 1000 times as much as an effective dose before a lethal overdose was reached.

Important safety information

Before taking medicinal cannabis, tell your doctor and pharmacist about all prescription and
over-the-counter medications and supplements you take or plan to take.
Tell your doctor immediately if you become severely ill and have some or all of these symptoms:
  • develop a rash, hives, swelling or have trouble breathing as these may be the symptoms of an allergic reaction; and/or
  • experience seizures or convulsions.
Common, immediate side effects of taking medicinal cannabis include:
  • loss of inhibition
  • spontaneous laughter
  • quiet and reflective mood
  • affected perception including sound, colour and other sensations
  • confusion, altered thinking and memory
  • possible mild paranoia
  • altered vision
  • reddened/bloodshot eyes
  • relaxation
  • sleepiness
  • reduced coordination and balance
  • increased heart rate
  • low blood pressure
  • increased appetite.
Indicators that the dose might be too high, or the formulation is not suitable for you include:
  • confusion
  • restlessness
  • excitement
  • hallucinations
  • anxiety or panic
  • detachment from reality
  • decreased reaction time
  • paranoia.
Rare side-effects, often associated with cannabis related allergens include:
  • nasal congestion
  • runny nose
  • post-nasal drip
  • sneezing.

Administration of medicinal cannabis

The two most common ways of taking medicinal cannabis are orally through extract oils or a sublingual wafer, or inhaling via vaporising dried plant material.

These two methods have different timings for onset and duration of action. Generally, vaporising of dried flowers is used for immediate relief of symptoms and oils are used for a baseline reduction in symptoms.

Smoking cannabis is not supported as a viable method of administration for medicinal cannabis. Smoking in general is harmful, and in the process of doing so much of the cannabinoid content can be lost, making specific dosing difficult to control.


When cannabis is inhaled, cannabinoids are absorbed from the lungs into the bloodstream within minutes. The cannabinoids are then transported to the brain, where they reach a peak concentration within 15 minutes. This correlates with the peak of the psychological and physiological effects that can be experienced.

The specific effects may vary between individuals based on a number of factors including – the heating of the cannabis, the number of inhalations, the time between inhalations, how long each inhalation lasts and your lung capacity.

Products designed for inhalation are often raw cannabis plant material that has been produced from plants with known and predictable cannabinoid content. The dried plant material is vaporised and inhaled using an approved and registered medical device. Your doctor will also need to obtain for you a vaporising device through the unapproved medical device access pathway.

Approval for using a medicinal cannabis product designed for inhalation is conditional on use with an appropriate approved and registered medical device.

Oral administration

When consumed orally, cannabinoids are absorbed from the digestive system into the bloodstream. This method of absorption is slower and more unpredictable than inhalation. As such, onset of action can take 60 to 180 minutes with the maximum effects experienced between two and three hours after ingestion.

While the potential timelines are broader, ingesting cannabis oils gives a slower onset of action and longer-lasting effects when compared to using a vaporiser. Variance in the onset and duration of action is due to several factors, including potency and quantity of cannabis oil consumed, your tolerance and mechanism of delivery. In addition, there are many patient variables that can influence the effects of ingested cannabis oil, such as the time of day or when you last ate.

Oils are made by extracting cannabinoids from cannabis plant material by exposing it to solvents. The oil is typically swallowed or put under the tongue using a dropper. Oils can also be put into capsules for swallowing like any other pill.

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