Safe usage tips

For smoked cannabis the two agree: NHS inform says to let previous doses wear off, "about 30 minutes", and the Alcohol and Drug Foundation says at least 30 minutes. For eaten cannabis they do not. NHS inform advises avoiding a repeat dose "for at least a couple of hours"; the Alcohol and Drug Foundation says "wait at least 4 hours before deciding whether to take more". Public Health Scotland's general drugs guidance names a third figure — "wait at least 2 hours" after a test dose of any drug.

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

NHS inform states that potency "can be hugely variable". The Royal College of Psychiatrists puts it plainly: "You usually won't be able to tell how strong cannabis is or the kind of effects it will have on you until after you have taken it." NIDA reports that between 1995 and 2022 the delta-9 THC potency of seized illegal cannabis quadrupled from 3.96% to 16.14%, and that flower and concentrates in dispensaries can exceed 40%. Public Health Scotland adds that drug testing is the only way to confirm what is in a sample, but "may not confirm the amount, potency or effects".

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

NHS inform states that inhaling any substance into the lungs can irritate the delicate tissue, and names tobacco, cannabis and vaping products together. Its harm-reduction guidance is to consider a vaporiser so tobacco is removed, to avoid holding smoke in the lungs or breathing too deeply, and to take short puffs; it notes that water pipes cause deeper inhalation and so increase the risk of lung damage. The Royal College of Psychiatrists adds that cannabis is often smoked with tobacco, "which carries the same health risks as smoking tobacco in cigarettes". WHO lists epithelial injury of the trachea and major bronchi from long-term cannabis smoking.

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

WHO states that performance on complex machinery "can be impaired for as long as 24 hours after smoking as little as 20 mg of THC in cannabis". The Alcohol and Drug Foundation puts the total duration of effects at up to 24 hours and says cannabis slows response times and affects coordination and balance, listing driving, operating heavy machinery and swimming together as activities affected. On the legal side, the UK government states it is an offence to drive over a specified blood level of certain drugs "even if they have not affected your driving", and police can screen for cannabis at the roadside.

Sources: WHO · Alcohol and Drug Foundation · GOV.UK

The Alcohol and Drug Foundation defines mixing as using more than one drug — including alcohol or medications — "at the same time, or one after another", and adds that what was taken in the previous 24 hours counts too. It 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 is to avoid mixing altogether because it "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)

The Alcohol and Drug Foundation states that psychoactive drugs can have varied effects depending on a person's mood — often called the "set" — and the environment they are in, the "setting". Public Health Scotland's harm-reduction guidance follows from that: because cannabis may worsen anxiety and paranoia in some people, its advice is to use only where one feels safe and with trusted people, and to avoid cannabis entirely if prone to anxiety or living with existing mental health problems.

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

Public Health Scotland's harm-reduction guidance for cannabis lists sharing separately from every other point: its advice is to avoid sharing joints, vapes, pipes and bongs with others. It is stated as a standalone measure alongside the guidance on vaporisers, inhalation depth and pipe materials, rather than as part of the smoking-technique advice.

Sources: NHS inform (Public Health Scotland)

This is a genuine, unresolved contradiction between two authorities. The Alcohol and Drug Foundation suggests choosing "full spectrum" products with a balance of other cannabinoids like CBD rather than THC-only products, saying this "may help reduce the risk of negative effects". The Royal College of Psychiatrists states the opposite: "Several studies have shown that when THC and CBD are inhaled together, CBD does not modify its effects in any way." Both are cited below; neither has been picked over the other here.

Sources: Alcohol and Drug Foundation · Royal College of Psychiatrists