Cannabis Research: Current State of Science

Scientific research on cannabis has experienced unprecedented momentum over the past two decades, evolving from a fringe area of pharmacology into one of the most active research fields in modern biomedicine. With advancing legalization in more and more countries, many of the legal hurdles that had hindered research for decades have fallen. The discovery of the endocannabinoid system in the 1990s provided the conceptual framework, and increasing clinical experience with medical cannabis has raised new questions. This article provides an overview of the most important current research areas, established findings and open questions.

## Neuroplasticity and Cognitive Effects

One of the most intensive research areas concerns the effects of cannabis on brain structure and function, particularly in adolescents and young adults. The human brain develops until approximately age 25, with the prefrontal cortex – responsible for planning, impulse control and decision-making – being among the last regions to mature.

**Findings in adolescent use:** Large cohort studies such as the ABCD Study (Adolescent Brain Cognitive Development), which has followed approximately 12,000 adolescents in the US since 2015, show subtle structural brain changes with early and regular cannabis use. A 2022 meta-analysis published in JAMA Psychiatry found minor but statistically significant differences in cortical thickness of the prefrontal cortex and hippocampal volume in adolescent cannabis users. However, the clinical significance of these differences is debated.

**Reversibility:** A central question is whether the cognitive effects of cannabis use are reversible. A 2022 meta-analysis in Neuropsychology Review concluded that most cognitive impairments fully resolve after an abstinence period of 72 hours to three weeks. Heavy long-term users showed subtle residual deficits in processing speed and verbal memory in some studies, which largely normalized after several months of abstinence.

**Age-dependent vulnerability:** Research consistently confirms that the risk of cognitive impairment correlates with the age of first use. Consumption before age 16 is associated with an increased risk of long-term effects, while onset in adulthood carries significantly lower risks.

## Pain Research

Chronic pain is the most common indication for medical cannabis, and pain research is producing increasingly nuanced results.

**Neuropathic pain:** The best evidence for the analgesic efficacy of cannabis exists for neuropathic pain. A 2018 Cochrane meta-analysis found moderate evidence that cannabinoid-based medicines relieve neuropathic pain better than placebo, with a Number Needed to Treat of approximately 5 to 6.

**Opioid-sparing effect:** Whether cannabis can reduce opioid consumption has attracted particular interest. Several observational studies from the US show that in states with legalized medical cannabis, opioid prescriptions and opioid-related deaths decline. A 2023 study in JAMA Internal Medicine found an average 14.5 percent reduction in opioid prescriptions in states with liberal cannabis legislation.

**Fibromyalgia:** A randomized controlled trial from the Netherlands (2021) showed that inhaled cannabis flowers with a mixed THC-CBD ratio significantly reduced fibromyalgia pain better than placebo. The effect size was moderate, and the study duration was limited, meaning long-term data are still lacking. A larger Israeli registry study following over 3,000 fibromyalgia patients using medical cannabis found that after six months, 81 percent reported significant improvement in pain symptoms, with 50 percent reducing or discontinuing other pain medications.

**Cancer pain:** The evidence for cannabis in cancer pain is mixed. While many patients report subjective improvement, randomized controlled trials have shown only modest benefits over placebo for cannabis as an adjunct to opioid therapy. The primary role of cannabis in oncology may be more as a multimodal symptom manager – simultaneously addressing pain, nausea, appetite loss and sleep disturbance – rather than as a primary analgesic.

## Anti-Inflammatory and Immunomodulatory Effects

The anti-inflammatory properties of cannabinoids are a highly active research area with potential applications in numerous disease areas.

**Neuroinflammation:** CBD has shown strong neuroprotective and anti-inflammatory properties in preclinical studies. In neurodegenerative diseases like Alzheimer's and Parkinson's, CBD shows promising effects in animal models. Clinical studies in humans are still in their early stages.

**Autoimmune diseases:** Cannabinoids are being researched as immunomodulating substances for rheumatoid arthritis, multiple sclerosis and lupus. Activation of CB2 receptors on immune cells can dampen the excessive immune response characteristic of autoimmune diseases.

**Bowel inflammation:** In Crohn's disease and ulcerative colitis, there are promising but still inconsistent data. An Israeli study (2018) showed that cannabis-rich oil promoted clinical remission in Crohn's disease but interestingly did not improve objective inflammatory markers. This suggests that cannabis may primarily improve symptoms (pain, appetite, sleep) without necessarily addressing the underlying inflammation – an important distinction for clinical practice.

**COVID-19 and cannabinoids:** The COVID-19 pandemic stimulated research into the anti-inflammatory potential of cannabinoids in viral infections. In vitro studies suggested that certain cannabinoids could reduce the cytokine storm associated with severe COVID-19. CBD in particular showed antiviral properties in cell culture experiments, apparently by modulating the host cell's stress response. However, these preclinical findings have not yet been translated into clinical application, and experts caution against overinterpreting laboratory results.

**Skin conditions:** CBD is increasingly being researched for inflammatory skin conditions including acne, eczema and psoriasis. The skin possesses its own endocannabinoid system with CB1 and CB2 receptors on keratinocytes, sebocytes and immune cells. Topical CBD preparations have shown anti-inflammatory and sebum-reducing effects in early clinical studies, though large-scale randomized trials are still needed.

## Psychiatric Research

### Cannabis and Psychosis

The question of whether cannabis can cause psychoses is one of the longest and most controversially discussed in cannabis research.

**Risk elevation:** Large epidemiological studies show that regular cannabis use increases the risk of developing psychotic disorders by a factor of 1.5 to 3. However, the absolute risk remains low: even with regular use, fewer than 5 percent of consumers develop a psychotic disorder.

**Genetic vulnerability:** Research has identified several genetic factors that modify the risk, most notably a polymorphism in the COMT gene (Val158Met). Newer genome-wide association studies have identified further risk variants.

**THC vs. CBD:** Psychosis research has shown that THC and CBD have opposing effects. THC can trigger acute psychotic symptoms, while CBD shows antipsychotic properties. A 2018 clinical study (McGuire et al., American Journal of Psychiatry) showed that CBD as adjunctive therapy in schizophrenia significantly reduced psychotic symptoms.

### Depression and Anxiety

The relationship between cannabis and affective disorders is bidirectional and dose-dependent. Low to moderate THC doses show anxiolytic and mood-elevating effects, while high doses can amplify anxiety and paranoia. CBD shows consistent anxiolytic effects across multiple clinical studies.

## Addiction Potential

**Epidemiology:** Approximately 9 percent of all persons who ever use cannabis develop a cannabis use disorder (CUD) over their lifetime. Among daily users, the proportion rises to approximately 25 to 30 percent. For comparison: the lifetime dependence rate is approximately 15 percent for alcohol, 32 percent for tobacco and 23 percent for heroin.

**Neurobiology:** Cannabis produces dependence through stimulation of the mesolimbic dopamine system – though considerably more weakly than opioids, stimulants or alcohol. Withdrawal symptoms after cessation of chronic use include irritability, sleep disturbances, reduced appetite and mild dysphoria, typically lasting one to three weeks.

**Risk factors** for developing cannabis dependence include early first use (before age 16), daily consumption, high-potency cannabis (high THC, low CBD content), concurrent tobacco use, psychiatric comorbidities (ADHD, anxiety disorders, depression) and family history.

**Treatment options:** For individuals who develop cannabis dependence, several evidence-based therapies are available. Cognitive behavioral therapy (CBT) has the strongest evidence, showing moderate effect sizes in reducing consumption. Motivational Enhancement Therapy (MET) is a brief approach targeting intrinsic motivation for change. Contingency management shows good results in studies. Pharmacotherapeutic approaches are still under investigation; no medication is yet approved specifically for cannabis dependence, though N-acetylcysteine and gabapentin have shown promising results in some studies.

## Cannabis and Driving Fitness

**Acute effects:** Cannabis acutely impairs reaction time, divided attention, lane tracking and decision-making. Driving simulators and on-road studies consistently show that cannabis intoxication worsens driving performance, though the effect size at moderate doses is typically less than at a blood alcohol concentration of 0.05%.

**THC blood level as a marker:** Unlike alcohol, where blood alcohol concentration provides a relatively reliable correlation with intoxication, THC blood levels are a poor predictor of current impairment. THC is lipophilic and stored in fatty tissue, so it can be detectable in blood days or weeks after last use without current impairment.

**The German threshold:** Germany long applied a THC threshold of 1 ng/ml in blood serum. The Threshold Commission recommended an increase to 3.5 ng/ml in 2024, based on scientific evidence that relevant impairment in occasional users typically begins at THC concentrations of 3 to 5 ng/ml.

## Long-Term Studies

**Respiratory health:** Regular cannabis smoking is associated with chronic bronchitis, increased coughing and sputum production. Surprisingly, most studies show no association between cannabis smoking and lung cancer, even with heavy long-term use. Vaporizer use eliminates most combustion by-products and considerably reduces respiratory burden.

**Cardiovascular risks:** Cannabis can acutely trigger tachycardia and orthostatic hypotension. Whether chronic cannabis use increases long-term cardiovascular risk remains debated.

**Long-term cognition:** The Dunedin Study (New Zealand) found a decline in IQ of approximately 6 points by age 38 in persons with chronic cannabis use from adolescence. However, this finding is controversially discussed as follow-up studies could not replicate the effect at the same magnitude and confounding factors (socioeconomic status, alcohol use, education) may not have been adequately controlled.

**Reproductive health:** Emerging research examines the effects of cannabis on fertility and reproductive health. THC can temporarily reduce sperm count and motility in men, though these effects appear reversible after cessation. In women, cannabis may affect ovulation timing and menstrual cycle regularity. The endocannabinoid system plays a role in embryo implantation, raising concerns about cannabis use around the time of conception.

## Methodological Challenges

Cannabis research faces unique methodological challenges that must be considered when evaluating the literature. First, the prohibition that prevailed in most countries for decades severely restricted research access. The US federal government, for example, long required that all cannabis research use material from a single government-licensed farm at the University of Mississippi – material that was often of lower quality and different composition than cannabis used by consumers.

Second, blinding in cannabis studies is inherently difficult. The psychoactive effects of THC make it obvious to participants whether they received active treatment or placebo. This expectation effect can significantly influence subjective outcomes such as pain or mood.

Third, the enormous chemical diversity of cannabis makes standardization challenging. A study using one specific cannabis variety or chemotype may not be generalizable to others. The scientific community is increasingly calling for studies that characterize not only THC and CBD content but the full cannabinoid and terpene profile of the material used.

## Open Questions and Research Gaps

Despite enormous progress, numerous questions remain open:

**Strain and chemotype-specific effects:** Most studies do not differentiate between various cannabis chemotypes (THC-dominant, CBD-dominant, balanced). Research needs to develop more nuanced designs that account for the influence of terpenes and rare cannabinoids.

**Sex-specific differences:** Women and men metabolize cannabis differently and show different sensitivity patterns. Women tend to be more sensitive to THC, develop tolerance faster and may have a higher dependence risk. Research must integrate sex-specific analyses more systematically.

**Pregnancy and breastfeeding:** Cannabis use during pregnancy is associated with an increased risk of low birth weight and possibly developmental delays. The evidence base is limited, however, as controlled studies are not possible for ethical reasons. Until more definitive data are available, all medical professional societies recommend abstinence during pregnancy and breastfeeding.

**Drug interactions:** Cannabis can influence the metabolism of other medications via the cytochrome P450 enzyme system. CBD in particular is a strong inhibitor of CYP2C19 and CYP3A4, which can alter the effects of anticoagulants, antiepileptics and immunosuppressants. Systematic interaction studies are urgently needed.

**Microdosing:** The concept of microdosing – the use of very low cannabis doses below the psychoactive threshold – is gaining interest. Initial studies suggest that microdoses may have therapeutic effects on pain and anxiety without causing relevant side effects or intoxication. Controlled studies are still pending.

**Cannabis and ageing:** As the population ages, the question of how cannabis affects older adults becomes increasingly relevant. Older adults are the fastest-growing demographic of cannabis users in many legalized jurisdictions. They tend to use cannabis primarily for medical reasons – pain, insomnia, anxiety – but their altered pharmacokinetics (slower metabolism, changed body composition) require adapted dosing guidelines that are not yet well established.

The advancing legalization worldwide will further improve research opportunities and is expected to answer many of these questions in the coming years.

## The Entourage Effect

One of the most fascinating and simultaneously contested concepts in cannabis research is the so-called entourage effect – the hypothesis that the totality of active compounds in cannabis (cannabinoids, terpenes, flavonoids) works synergistically and offers greater therapeutic benefit than isolated individual substances.

The term was coined in 1998 by Raphael Mechoulam and Shimon Ben-Shabat and has significantly influenced the debate about full-spectrum extracts versus isolated cannabinoids. Clinical observations support the hypothesis: many patients report that cannabis flowers or full-spectrum extracts work more effectively than pure THC or CBD. In a study on epilepsy patients, CBD-rich cannabis extract showed better seizure control at lower doses than isolated CBD.

However, the scientific evidence for the entourage effect remains limited. Critics argue that many terpenes in cannabis are present in such low concentrations that a pharmacologically relevant effect is unlikely. Research must proceed with rigorous, placebo-controlled studies that systematically compare different cannabinoid-terpene combinations.

## Cannabinoids and Cancer

The question of whether cannabinoids have direct anti-tumour effects is one of the most intensively researched and simultaneously most frequently misunderstood topics in cannabis research. In cell culture and animal experiments, THC, CBD and other cannabinoids have demonstrated several anti-tumoral mechanisms: induction of apoptosis in tumour cells, inhibition of angiogenesis, inhibition of cell migration and metastasis, and modulation of the immune system favouring an anti-tumoral immune response.

These preclinical data are promising but must be interpreted with extreme caution. What works in a Petri dish or in mice cannot automatically be transferred to humans. One of the few randomized studies – a British phase II trial in glioblastoma patients (2021) – showed that the combination of THC and CBD in addition to standard therapy (temozolomide) extended median survival by several months. These results are encouraging but the study was small and must be confirmed by larger studies.

It is crucial to distinguish between the palliative use of cannabis in oncology (treating pain, nausea, appetite loss) and a potential anti-tumoral therapy. The former is well documented and established. The latter is an active research area that gives hope but cannot yet show clinically validated results. Patients should be strongly advised against abandoning conventional cancer therapies in favour of cannabis.

## The Future of Cannabis Research

The future of cannabis research will be shaped by several trends:

**Personalized cannabinoid therapy:** The increasing availability of genetic tests could enable prediction of individual responses to cannabinoids. Patients with certain COMT gene variants or differences in cannabinoid receptor genes could benefit from tailored cannabinoid therapies.

**Next-generation synthetic cannabinoids:** The pharmaceutical industry is increasingly developing synthetic cannabinoids that selectively bind to specific receptors and trigger specific therapeutic effects without the psychoactive side effects of THC. CB2-selective agonists for inflammation and pain, as well as peripherally acting CB1 agonists that do not cross the blood-brain barrier, are promising approaches.

**Real-world evidence:** With the increasing adoption of medical cannabis, the volume of care data captured through registry studies and electronic patient records is growing. This real-world evidence complements data from controlled studies and provides insights into long-term efficacy and safety under everyday conditions.

**Interdisciplinary networking:** Cannabis research was long a niche field. It is increasingly connecting with broader biomedical research – in neuroscience, immunology, oncology and psychiatry. This networking accelerates knowledge gain and improves research quality.

## Summary: What We Know and What We Don't

Cannabis research has made enormous progress. Established findings include therapeutic efficacy for neuropathic pain, spasticity and certain epilepsy forms; the elevated psychosis risk in genetically vulnerable adolescents; the reversibility of most cognitive effects in adults; and the moderate but real addiction potential. At the same time, central questions remain open regarding long-term effects of different consumption patterns, optimal dosing, sex-specific effect profiles and the role of rare cannabinoids and terpenes.