Cannabis products are getting more potent and easier to buy around the world. Yet little is known about how bigger doses of tetrahydrocannabinol (THC), the main active ingredient in cannabis, change what people feel. Our new research found that larger doses generally make people feel more high, but not in a straight line. Doubling the dose does not double the feeling of intoxication.
This leaves a simple but important question for first-time and returning users: how will the amount of THC I take make me feel?
Cannabis laws have changed in many countries, and in some cases this has led to increased access to and diversity of products, as well as some increases in first-time cannabis users. Meanwhile, the amount of THC in cannabis products has been rising around the world, whether or not cannabis is legal.
THC is the part of cannabis that changes how you think and feel, and it produces both positive and negative effects. It does this by acting on receptors in the brain, mainly one called CB1. These receptors are part of the endocannabinoid system, which helps keep the body stable and protected.
First-time and inexperienced users are the most likely to take too much by accident and have unwanted effects, such as anxiety and paranoia. The huge range of products on the market makes it hard for them to judge how much is too much.
For our study, published in the journal Neuroscience and Biobehavioral Reviews, we combined evidence from 41 studies looking at the immediate effects of THC on how people feel. It included 1,020 participants who used cannabis less than once a week. In these experiments, people were given known doses of THC, and their reports were compared with those after a placebo (a dummy treatment), because simply believing you have taken THC can cause a mild high.
So how does THC change the way people feel? Overall, people who rarely used cannabis felt more high, anxious and tired, and less alert and content, than they did on the placebo.
We looked at the effects of inhaling a “Standard THC Unit” (5mg of THC), which is the proposed cannabis version of alcohol “units” or “standard drinks”. On a 100-point scale, after subtracting the effects of the placebo, a Standard THC Unit raised “high” by 34 points, “anxiety” by 14, “tired” by 26 and “calm” by five. It lowered “content” by 71 points and “alert” by 40.
Doubling the dose from 5mg to 10mg raised the intensity of the high only from 34 to 39 points – an increase of just 15%. This is common in pharmacology – the study of how drugs work – where bigger doses often bring smaller and smaller extra effects once a certain level is reached. (There was no change in anxiety, tiredness or alertness compared with the 5mg dose.)
For inexperienced users, this suggests that taking more THC can strengthen the effects they want, but the benefits may level off fairly quickly. Each extra dose adds less than the last.
Our results support the classic advice for reducing the risks of drug use: “start low, go slow”. Begin with a low dose and wait to feel the effects before taking more. The pleasurable effects may level off fairly quickly, but larger doses can still cause unpleasant experiences.
The cost of taking more THC
The brain adapts readily – when THC repeatedly activates the cannabinoid receptors, the brain temporarily reduces how many are available. You then need more THC to get the same high as before. This is known as tolerance, and it makes getting high more expensive.
The endocannabinoid system also helps the brain cope when it is under strain. It helps regulate sleep, appetite, pain, reward and, in particular, mood and stress. Research on this system suggests it is a bad idea to weaken it with heavy cannabis use. Long-term studies have found an increased risk of addiction and mental health problems in vulnerable people from frequent use of high-potency cannabis.
Our study has limits, though. Combining data from several studies is a strength of the meta-analysis approach, which pools the results of many studies, but the studies we included differed in many ways. They involved different participants, types of THC (some synthetic, some herbal), ways of taking it (vaping or joints), ways of measuring consumption and settings.
A next step is to test our findings directly in a controlled study where the same participants receive the most commonly used THC doses. Until then, our findings offer a simple message for people with little experience of cannabis: the high rises quickly at low doses, but extra THC soon adds less and less, which backs the advice to start low and go slow.