Know the risks

Smoked, effects start within a minute or two. Eaten, onset is delayed — Talk to FRANK says up to an hour, Australia's Alcohol and Drug Foundation says 30 minutes to 2 hours. The Alcohol and Drug Foundation puts the peak at 1 to 4 hours, and Talk to FRANK adds that there may be a few more hours before the effects wear off completely. NHS inform describes eating cannabis as giving a much longer duration and a more delayed onset than smoking.

Sources: Talk to FRANK · Alcohol and Drug Foundation · NHS inform (Public Health Scotland)

The delay is the mechanism. NIDA states that edibles "typically take longer to show effects. People may use more of a product as a result, increasing the likelihood of serious negative health effects." Talk to FRANK adds that the amount of cannabis in these products can vary greatly. Public Health Scotland advises against a repeat dose for at least a couple of hours; the Alcohol and Drug Foundation says at least 4 hours after eating cannabis, and at least 30 minutes after any other route.

Sources: NIDA · Talk to FRANK · NHS inform (Public Health Scotland) · Alcohol and Drug Foundation

WHO states that human performance on complex machinery "can be impaired for as long as 24 hours after smoking as little as 20 mg of THC in cannabis". Australia's Alcohol and Drug Foundation likewise puts the total duration of effects, smoked or eaten, at up to 24 hours.

Sources: WHO · Alcohol and Drug Foundation

In Great Britain it is an offence to drive with more than a specified level of certain drugs in the blood "even if they have not affected your driving" — the UK government's own wording. Police can screen for cannabis at the roadside, and the maximum penalty for causing death by careless driving under the influence of drugs is life imprisonment.

Sources: GOV.UK

Talk to FRANK states that driving while stoned "can double your chances of having a serious, or even fatal car crash". NIDA reports cannabis as the drug most frequently found in the blood of crash-involved drivers, and that several analyses found increased crash risk — but also cites a US National Highway Traffic Safety Administration study that "found no significant increased crash risk attributable to cannabis use". The size of the risk is genuinely contested.

Sources: Talk to FRANK · NIDA

NIDA reports that studies "have estimated that 22% to 30% of people who use cannabis have" cannabis use disorder. NIDA identifies frequency of use as the strongest predictor, with family history of drug use and duration of use also playing a role, and further cites one estimate that 12.1% of people who frequently use cannabis experience cannabis withdrawal.

Sources: NIDA

Public Health Scotland states that people using cannabis every day over a prolonged period may develop a tolerance to the effects and need more to get the same result. The Royal College of Psychiatrists reports that withdrawal symptoms "usually start within one or two days after cannabis was last used", "can last for around two weeks", and are most intense during the first week — commonly insomnia, anxiety, irritability, low mood, restlessness and decreased appetite.

Sources: NHS inform (Public Health Scotland) · Royal College of Psychiatrists

The Royal College of Psychiatrists states that "the younger you are when you start using cannabis, the more likely you are to develop psychosis", and that using stronger forms or using it regularly also raises the risk — explaining the age effect by the brain still developing. NIDA links cannabis to earlier onset of psychosis in people with genetic risk factors for psychotic disorders including schizophrenia, and to worse symptoms in those who already have them, an association "especially strong in young males compared to females". Public Health Scotland names family history, heavy use from a young age over a long period, and high-potency cannabis as the risk factors. The Alcohol and Drug Foundation notes the brain continues developing into a person's mid-twenties.

Sources: Royal College of Psychiatrists · NIDA · NHS inform (Public Health Scotland) · Alcohol and Drug Foundation

NIDA describes CHS as nausea, vomiting and abdominal pain following long-term, heavy cannabis use: "It can recur and often requires medical attention. People sometimes try to temporarily relieve their symptoms by taking hot showers or baths. However, CHS only resolves when a person stops using cannabis completely." The Alcohol and Drug Foundation describes it as rare, and most commonly linked to heavy, long-term smoking of high-THC cannabis.

Sources: NIDA · Alcohol and Drug Foundation

NIDA links cannabis use in pregnancy with lower birth weight, preterm birth, hospitalisation and death within one year of birth, and reports that the American College of Obstetricians and Gynecologists counsels against use while trying to conceive, during pregnancy and while breastfeeding. WHO states that cannabis used during pregnancy is associated with impairment in fetal development leading to a reduction in birth weight. The Alcohol and Drug Foundation notes that THC passes into breast milk and that the effects on the baby are not yet well understood.

Sources: NIDA · WHO · Alcohol and Drug Foundation

The Alcohol and Drug Foundation states that combining THC with alcohol and other depressants can increase drowsiness and sedation, and that mixing with alcohol can also cause nausea and vomiting. Public Health Scotland's general guidance on drugs is to avoid mixing at all — including with alcohol and medicines — because mixing "can cause unexpected and unpredictable results", and names it a major risk factor in drug-related deaths in Scotland.

Sources: Alcohol and Drug Foundation · NHS inform (Public Health Scotland)