THC

Delta-9-tetrahydrocannabinol (THC) is the primary psychoactive compound in medicinal cannabis legally prescribed in New Zealand. It is the molecule responsible for the “high” associated with cannabis, but in a regulated pharmaceutical context it is used for pain, nausea, appetite stimulation and sleep disturbance. Under the Misuse of Drugs Act 1977, THC is a controlled drug; legal access is only through the Medicinal Cannabis Scheme, via a prescription from a medical practitioner and a product approved by the Ministry of Health (Medsafe, 2020).

What THC is and how it works

THC (delta-9-tetrahydrocannabinol) is the main psychoactive cannabinoid in cannabis. It binds to CB1 receptors in the brain and central nervous system, producing the “high” and most of the symptom-relieving actions. It is measured in mg/g or as a percentage of dry weight on product labels. The NZ Formulary lists THC as a controlled drug available with a valid prescription, and Medsafe regulates preparations under the Misuse of Drugs Act 1975 (New Zealand Ministry of Health, 2020).

THC acts on the endocannabinoid system, but it is not a simple “painkiller.” Its actions vary by dose, route, individual metabolism, and the ratio of THC to CBD. At low doses, it can reduce nausea, stimulate appetite, and relax muscles. At higher doses, it can produce sedation, altered perception, and anxiety. This is why the clinical guidance is to start low and go slow, and why preparations are labelled with exact THC and CBD amounts in mg/g.

THC is a partial agonist at CB1 and CB2 receptors, with CB1 in the brain driving psychoactivity; effects appear within minutes when inhaled and within 30-90 minutes when ingested. THC is metabolised by CYP2C9 and CYP3A4; it is highly lipophilic and accumulates in fat, leading to a long elimination half-life of 1-2 days for occasional use and 7-13 days for chronic use.

Psychoactive actions and impairment

THC is the main psychoactive cannabinoid in medicinal cannabis. Its actions are dose-dependent and vary by person.

• Euphoria and relaxation: THC binds to CB1 receptors in the brain, producing the “high” that is often described as euphoria or relaxation. In clinical contexts, this can be useful for mood elevation in palliative care, but it is also the main reason for impairment.
• Altered perception: time distortion, heightened sensory awareness, and changes in thinking can occur at therapeutic doses; these are not always unpleasant but can interfere with work, driving, or caregiving.
• Sedation: higher doses are more sedating; this is often the intended action for sleep, but it can impair daytime function.
• Appetite stimulation: THC is well-documented to increase appetite, which is why it is used in cachexia and HIV-related wasting (Ministry of Health, 2023).
• Anxiety or paranoia: at higher doses, THC can trigger acute anxiety, paranoia, or panic, especially in people with no tolerance or a history of anxiety disorders.

What to expect: psychoactive and adverse reactions

• Psychoactive actions: euphoria, relaxation, altered time perception, increased appetite, sensory changes; these are the intended actions in some cases, but are dose-dependent and unpredictable for new users.
• Impairment: THC impairs short-term memory, reaction time, coordination, and divided attention; impairment is greatest during the first 1-2 hours after inhalation and 2-4 hours after oral use, but some effects persist longer.
• Adverse reactions: dry mouth, red eyes, increased heart rate, orthostatic hypotension, dizziness, anxiety, paranoia, short-term memory impairment, sedation, increased appetite. At high doses, can trigger panic attacks or psychosis in vulnerable individuals. Long-term heavy use is linked to cannabis use disorder, cognitive effects in adolescents, and cannabinoid hyperemesis syndrome (rare).
• Dependence: THC is dependence-forming. Regular daily use can lead to tolerance and withdrawal (irritability, insomnia, appetite loss) on cessation; the risk is higher with high-THC preparations and inhaled routes. The NZ Formulary notes that dependence risk is lower with prescribed use than with recreational use, but it is not zero.

Driving and workplace cautions

Read this section before you use any THC-containing preparation. The legal risks are immediate and do not depend on whether you feel impaired.

Driving law

Under New Zealand law, it is an offence to drive with a specified level of THC in your blood. The limit is 2 ng/mL for drivers 20 years and over, and 1 ng/mL for those under 20 (Land Transport Act 1998, as amended in 2022). There is no “medical exemption” for THC; even a prescribed patient can be convicted if they exceed the limit. This is the single most important legal fact for patients who drive.

In New Zealand, it is illegal to drive with THC in your blood above the specified limit regardless of prescription. Even if a patient feels unimpaired, THC can be detected for days after use, and a roadside test can result in a charge. The NZ Formulary advises patients not to drive while using THC, and the Ministry of Health recommends a 4-hour wait after smoking or vaping, and 8 hours after oral use, though this is not a legal safe harbour.

Separately, driving with any amount of THC in your system is illegal if you are impaired. If a police officer considers you impaired, you can be required to do a roadside impairment test and a blood test. Having a prescription does not exempt you from impaired driving laws.

Workplace policies

Employers can set workplace drug policies that are stricter than the criminal law. Patients using prescribed THC may still be dismissed or disciplined if they test positive, and there is no general employment-law exemption for prescribed cannabis. The Human Rights Commission has stated that an employer may treat the use of prescribed medication as a breach if it impairs the employee’s ability to perform the job safely.

Obtaining THC legally in New Zealand

The prescription process

You need a prescription from a medical practitioner; GPs, specialists, and nurse practitioners can prescribe. Specialists may prescribe for any condition, but most GPs limit to the conditions in the NZ Formulary (chronic pain, cancer-related nausea/vomiting, spasticity in MS). Prescribing is a shared decision; the doctor must explain risks and benefits, and the patient must give informed consent. The consultation is not funded by the government, so costs vary.

Only a medical practitioner can prescribe; nurse practitioners and pharmacists cannot initiate. The prescriber must be satisfied that conventional treatment has failed or is inappropriate. Prescribing is not limited to specialists; any doctor can do it under the Medicinal Cannabis Scheme. The prescriber must apply for a Section 29 approval from the Ministry of Health if the preparation is not already approved under the scheme.

Available preparations

Legal preparations are GMP-certified, and the main forms are dried flower for inhalation (usually via a vaporiser) and oral oils. Flower has a faster onset but shorter duration; oils are slower but last longer. Preparations are labelled by THC and CBD content in mg/g or %. All preparations must be approved by the Ministry of Health and meet GMP standards. Common forms are dried flower for inhalation (via a vaporiser, not smoking), oral oils, capsules, and sublingual sprays. Raw flower is not a “legal high” and cannot be smoked; it is prescribed and dispensed through a pharmacy.

Regulation and legal status

The Medicinal Cannabis Scheme was created under the Misuse of Drugs (Medicinal Cannabis) Amendment Act 2018 and fully commenced in April 2020. The Ministry of Health runs the scheme; Medsafe sets quality standards; the Medicinal Cannabis Agency issues licences. Preparations must meet GMP standards and be approved under the scheme. The NZ Formulary lists cannabis preparations for prescribing. The Misuse of Drugs (Medicinal Cannabis) Amendment Act 2018 legalised access via prescription; since the MCS started on 1 April 2020, preparations meeting GMP standards can be prescribed and dispensed through normal pharmacy channels. The raw flower, while available, is prescribed and dispensed through a pharmacy, not a dispensary.

Patients cannot legally import raw flower or unapproved preparations for personal use; the only legal route is a prescription filled at a New Zealand pharmacy.

What the research says for specific conditions

Chronic pain

A 2023 systematic review in The BMJ found that cannabis-based medicines (including THC-dominant preparations) produced a modest improvement in pain (≥30% reduction) compared with placebo, but the effect was small and accompanied by a higher risk of dizziness, sedation, and confusion; the authors rated the certainty of supporting data as moderate for pain, but low for most other outcomes. A 2021 systematic review in the Journal of the American Medical Association (JAMA) found moderate-quality data that cannabis improves pain, but the effect size is small and inconsistent; many patients experience no benefit. A 2023 systematic review by the American Medical Association (AMA) found that cannabis is associated with a small-to-moderate improvement in pain, but the quality of data is low-to-moderate; the review noted that adverse events (dizziness, sedation, confusion) were common and led to withdrawal in ~10% of patients.

Chemotherapy-induced nausea and vomiting (CINV)

The 2015 Cochrane review found that cannabinoids (including THC) are likely effective, but the evidence base is older and does not reflect modern antiemetic regimens; the review’s authors noted the need for newer trials. A 2023 systematic review in JAMA found that cannabis-based medicines reduced nausea and vomiting compared to placebo, but the effect size was small and the quality of data was low.

Spasticity in multiple sclerosis

A 2021 Cochrane review found that nabiximols (a THC:CBD oromucosal spray) probably improves muscle spasticity at 12 weeks, but the effect is modest; the review noted that most patients do not achieve a 30% or greater reduction in spasticity.

Epilepsy

The 2018 FDA approval of Epidiolex (pure CBD) for Dravet and Lennox-Gastaut syndromes is the strongest evidence; THC alone is not recommended for epilepsy.

Anxiety, depression, PTSD and sleep

A 2023 systematic review in The Lancet Psychiatry found limited data for cannabinoids in treating anxiety disorders, with a small effect size and high risk of bias in most studies. The same 2023 Lancet review found no high-quality data that cannabinoids improve depression; some studies suggest potential harm. A 2021 systematic review in the Journal of Clinical Psychopharmacology found insufficient data to recommend cannabinoids for PTSD. A 2022 systematic review in the Journal of Sleep Research found that cannabis may reduce sleep latency but not improve sleep maintenance; the data is weak and mixed.