THC vs CBD: What Is the Difference?

Two chemicals from one plant

The cannabis plant contains around 130 related chemicals called cannabinoids. Two of them do most of the work: THC (tetrahydrocannabinol) and CBD (cannabidiol).¹

Both act on the body's endocannabinoid system, a signalling network that helps regulate mood, memory, sleep, appetite and the immune system.³ They act on it in different ways. THC is the main mind-altering ingredient in cannabis and partly switches on the system's two main receptors. CBD does not act on those receptors in the same way.²

THC and CBD at a glance

What is THC?

THC is the chemical that makes a person feel high.¹ It can make people feel relaxed, happy, hungry or drowsy. It can also make some people anxious or paranoid.⁴

How fast it works depends on how it is taken. Smoked or vaped, the effects are immediate and peak within 15 to 30 minutes. Swallowed, they start within 30 minutes to 2 hours and peak at 1 to 4 hours.⁵

The risks of THC

• Memory and thinking. In a trial of 46 healthy volunteers, a 10 mg dose of inhaled THC impaired memory and brought on short-lived psychotic-like symptoms.⁶
• Dependence. In a review of 21 studies, 22 per cent of people who used cannabis had cannabis use disorder. Among young people using it weekly or daily, the risk of dependence rose to 33 per cent.⁷
• Psychosis. In a study across 11 sites, most of them in Europe, daily cannabis use was linked to about three times the odds of a psychotic disorder, and daily use of high-potency cannabis to nearly five times.⁸
• Teenagers. Regular use in adolescence roughly doubles the risk of leaving school early, and of cognitive impairment and psychosis in adulthood.⁹
• Who should not be prescribed it. An Australian review for GPs says products containing THC should not be prescribed to people with angina or a previous heart attack, or to people with a personal or family history of psychosis.¹⁰

What is CBD?

CBD does not cause a high.¹ Most of it is extracted from hemp, a type of cannabis plant that is high in CBD and low in THC.³

Its side effects are milder than those of THC. A pooled analysis of 12 trials concluded that CBD is well tolerated, with reduced appetite, diarrhoea and sleepiness more common than on placebo. It can interact with other medicines.¹¹

The best evidence for CBD is in rare childhood epilepsies, at high doses. Its benefits in other conditions become clearer at doses of 300 mg a day or more, and it is uncertain whether lower doses do much.¹² Our guide to CBD oil in Australia goes through this in detail.

Does CBD cancel out THC?

It is often said that CBD takes the edge off THC, and that it may help control the high.¹ The best test of that idea so far does not support it.

Researchers gave 46 volunteers cannabis containing 10 mg of THC with no CBD, or with 10, 20 or 30 mg of CBD. THC impaired memory and caused psychotic-like symptoms, and no dose of CBD changed that. They found no evidence that CBD protects against the short-term harms of cannabis at the ratios most common in medicinal and recreational products.⁶

THC and CBD in medicinal cannabis

Prescribed products contain THC, CBD or both, and the label shows how much of each.¹ In a 2020 Australian survey, people with a prescription described their product as mainly THC (26 per cent), equal THC and CBD (40 per cent) or mainly CBD (31 per cent).¹³

Only two cannabis medicines are registered in Australia: a mouth spray with equal parts THC and CBD for muscle stiffness in multiple sclerosis, and a CBD-only liquid for rare childhood epilepsies. More than 99 per cent of what is prescribed is unapproved.¹⁴ Unapproved means the medicines regulator has not assessed the product for safety, quality or effectiveness.²⁰

See medicinal cannabis in Australia for how prescribing works and what the evidence shows for different conditions.

The law

Both THC and CBD are medicines under Australian law. Products containing either one have to be prescribed by a doctor or nurse practitioner and dispensed by a pharmacy.¹⁵

In December 2020 the regulator decided that low-dose CBD could be sold by pharmacists without a prescription, but only once a product had been approved and registered. None had been when the decision was made.¹⁶

Cannabis that has not been prescribed is illegal to possess in most of Australia.⁴ See is cannabis legal in Australia?

Driving and drug tests

THC impairs driving. In an on-road study, cannabis containing THC made drivers weave more in their lane 40 to 100 minutes after use, while cannabis that was mostly CBD was no different from placebo.¹⁷ A review of 80 studies estimated that most driving-related skills recover within about 5 hours of inhaling a 20 mg dose of THC, and that oral THC may take longer.¹⁸

Roadside saliva tests look for THC, and they detect its presence, not impairment.¹⁹ You can be charged if THC is found in your system while driving, whether or not it was prescribed. Some products that do not list THC as an ingredient still contain traces of it.¹⁵

Our guide to how long cannabis stays in your system covers detection times.

When to talk to a doctor

• You are considering a cannabis medicine and want to know whether there is evidence for your condition
• You use cannabis and have noticed changes in your mood, memory or motivation
• You or a family member has had psychosis, and you use cannabis⁴
• You are finding it hard to cut down

References

1. healthdirect. Medicinal cannabis.
2. Pertwee RG. The diverse CB1 and CB2 receptor pharmacology of three plant cannabinoids: delta9-tetrahydrocannabinol, cannabidiol and delta9-tetrahydrocannabivarin. Br J Pharmacol. 2008;153(2):199-215.
3. Alcohol and Drug Foundation. CBD (Cannabidiol).
4. healthdirect. Cannabis (marijuana).
5. Alcohol and Drug Foundation. Cannabis (THC).
6. Englund A, Oliver D, Chesney E, et al. Does cannabidiol make cannabis safer? A randomised, double-blind, cross-over trial of cannabis with four different CBD:THC ratios. Neuropsychopharmacology. 2023;48(6):869-876.
7. Leung J, Chan GCK, Hides L, et al. What is the prevalence and risk of cannabis use disorders among people who use cannabis? a systematic review and meta-analysis. Addict Behav. 2020;109:106479.
8. Di Forti M, Quattrone D, Freeman TP, et al. The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI): a multicentre case-control study. Lancet Psychiatry. 2019;6(5):427-436.
9. Hall W. What has research over the past two decades revealed about the adverse health effects of recreational cannabis use?. Addiction. 2015;110(1):19-35.
10. Arnold JC. A primer on medicinal cannabis safety and potential adverse effects. Aust J Gen Pract. 2021;50(6):345-350.
11. Chesney E, Oliver D, Green A, et al. Adverse effects of cannabidiol: a systematic review and meta-analysis of randomized clinical trials. Neuropsychopharmacology. 2020;45(11):1799-1806.
12. Arnold JC, McCartney D, Suraev A, et al. The safety and efficacy of low oral doses of cannabidiol: An evaluation of the evidence. Clin Transl Sci. 2023;16(1):10-30.
13. Lintzeris N, Mills L, Abelev SV, et al. Medical cannabis use in Australia: consumer experiences from the online cannabis as medicine survey 2020 (CAMS-20). Harm Reduct J. 2022;19(1):88.
14. Alcohol and Drug Foundation. Medicinal cannabis products.
15. Therapeutic Goods Administration. Medicinal cannabis: information for consumers.
16. Therapeutic Goods Administration. Over-the-counter access to low dose cannabidiol.
17. Arkell TR, Vinckenbosch F, Kevin RC, et al. Effect of Cannabidiol and Δ9-Tetrahydrocannabinol on Driving Performance: A Randomized Clinical Trial. JAMA. 2020;324(21):2177-2186.
18. McCartney D, Arkell TR, Irwin C, et al. Determining the magnitude and duration of acute Δ(9)-tetrahydrocannabinol (Δ(9)-THC)-induced driving and cognitive impairment: A systematic and meta-analytic review. Neurosci Biobehav Rev. 2021;126:175-193.
19. Alcohol and Drug Foundation. Roadside drug testing.
20. Therapeutic Goods Administration. Medicinal cannabis hub.