As the data continue to show, the incidence of breast, testis, and other cancers are on the rise in adolescent and young adults (AYAs) between the ages of 15 and 39, with increases of 30% over the last 4 decades.1 And while potential explanations for this rapid increase have included obesity, sedentary lifestyle, chest radiation, alcohol consumption, and hereditary predisposition, a major new study is offering another potential clue: the increasing use of cannabis among AYAs.
The study by Rebecca H. Johnson, MD, Medical Director of the Adolescent and Young Adult Oncology Program at Mary Bridge Children’s Hospital, MultiCare Health System in Tacoma, Washington, and her colleagues, compared breast and testis cancer incidence trends in American states that had or had not legalized the use of cannabis, and also compared cancer incidence trends vs cannabis use in Canada.
Rebecca H. Johnson, MD
Using annual cancer incidence data between 2000 and 2019 from the U.S. Surveillance, Epidemiology, and End Results (SEER) program, and for Canada, the Institute for Health Metrics and Evaluation (IHME), the study evaluated breast cancer incidence trends in females 20–34 years old, and testis cancer in males 15–39 years old. During this time interval, cancer registries in cannabis-legalizing states compared with non-legalizing states had a 26% vs 17% increase in breast cancer and 24% vs 14% increase in testis cancer in this population. Canada reported even greater increases in both breast cancer and testis cancer incidence in young adults than the United States, with breast cancer rates rising by 35%, and testis cancer surging by 83%, during the same time period,2 likely due to Canada’s earlier and broader legalization of cannabis use, according to Dr. Johnson.
“The evidence from our study implicates cannabis as a potential etiologic factor contributing to the rising incidence of breast cancer in young females and testis cancer in older adolescent and young adult males, and in most races and ethnicities,” said Dr. Johnson. “And, disturbingly, it appears that the carcinogenic effect of cannabis on AYAs is rapid, leading to cancer within just a few years of exposure to this drug. We need additional research into the role of cannabis as a contributing factor in the rising rates of these cancers in AYAs.”
Previous studies have suggested that cannabis “is a more important carcinogen than tobacco and alcohol, and fulfills epidemiological qualitative and quantitative criteria for causality for 25/41 cancers.”3
In this wide-ranging interview with The ASCO Post, Dr. Johnson discussed how cannabis may be contributing to the rise in cancer in AYAs and how the drug may impede cancer treatment effectiveness.
Understanding the Role of Cannabis on Cancer Development
The findings from your study on cannabis’ effect on adolescents and young adults and the increasing rates of breast and testis cancers in this population is extremely troubling. Please talk about the role cannabis may have in accelerating the development of these cancers in AYAs.
We wanted to test the hypothesis that the increasing incidence of these cancers in AYAs correlates with increasing use of cannabis at the population level. We focused on AYAs because they use cannabis more frequently than all other age groups in the United States.
Between 2000 and 2019 there was a rapid rate of legalization of cannabis across the United States, which is even greater now than it was during the period of our study. Currently, 47 states, the District of Columbia, and three territories, including Guam, Puerto Rico, and the U.S. Virgin Islands, allow the use of cannabis for medical purposes, and 24 states, plus the District of Columbia and two territories, Guam and North Mariana Islands, permit the use of recreational cannabis,4 so there’s nearly universal use of the drug throughout the country. And the drug’s commercial potency has increased as well, from just 4% tetrahydrocannabinol (THC) in the 1990s to over 14% in 2019, so the effects from exposure to the drug is greater.5
Our first question was “What is happening in the United States to the cancer rates in AYAs in states that have legalized the use of cannabis vs those that have not?” We analyzed SEER data on 20- to 34-year-old females and 15- to 39-year-old males to evaluate breast and testis cancers, respectively. Our next question was “How do these statistics compare with Canada’s?” We analyzed Canada’s IHME database to obtain the information.
Canada approved cannabis nationwide for medicinal purposes in 2001 and for recreational use in 2018. So, the drug’s been legally available there longer than in the United States, which may account for greater increases in both breast and testis cancers in AYAs in that country.
Cannabis is a known endocrine disrupter, potentially acting on reproductive organs via cannabidiol receptor 1 (CB-1), which is present in both breast and testis tissue. We also know that a morphologic change occurs when breast adipocytes in adolescent mice come in contact with cannabis: the adipocytes decrease in both number and size. So, something is happening to the breast at a uniquely sensitive period in adolescent development that may be contributing to the risk of breast cancer. Obesity could also increase both breast adipocyte size and the associated number of cannabinoid receptors, potentially escalating the risk of cannabis-induced breast carcinoma.
Cannabis exposure decreases luteinizing hormone and its activation of several types of gonadotropin receptors. Activation of gonadotropin-releasing hormone 2 (GNRH2) protects against breast cancer. So, potentially, there is a plausible role for hormone regulation of carcinogenesis by cannabis. We need more basic studies in humans to understand the exact correlation between cannabis use and the development of breast and testis cancers.
Australian researchers have also reported that in the United States and Europe multiple types of cancer are associated with cannabis use, including breast, testis, ovarian, and prostate cancers, and commented that cannabis is a more potent carcinogen than alcohol and tobacco.3
Smoking vs Ingesting THC-infused Edibles in Cancer Risk
Does the frequency and form of cannabis matter in increasing cancer incidence rates in AYAs? For example, is smoking cannabis rather than chewing THC-infused edibles make a difference in cancer risk?
Our study didn’t investigate that question. Other studies have noted a connection between cannabis use and the development of oropharyngeal cancers. When smoked, cannabis contains carcinogens similar to tobacco. There are few studies on the carcinogenic potential of cannabis ingested via food.
Investigating Why Cannabis May Stimulate Breast Cancer
In your study, cannabis use was most strongly correlated with the development of hormone-receptor negative breast cancer, why?
We don’t know the answer to that question, and are continuing to investigate this finding. Prognosis is dramatically worse for patients with triple-negative breast cancer compared to the hormone-receptor-positive subtypes, so the trend is concerning.
Counseling Patients to Avoid Cannabis During Treatment
Your study results show that cannabis may reduce cancer treatment effectiveness. Do you know why treatment might be impacted by the drug?
We looked at a prospective observational study of newly diagnosed patients with metastatic breast cancer, which showed a statistically significantly shorter median time to progression, 3.4 months vs 13.1 months, and median overall survival, 6.4 months vs 28.5 months, for cannabis users compared with nonusers.2
These statistics are very worrisome if borne out in larger randomized trials, because young patients are routinely using cannabis to manage symptoms and treatment side effects. While we don’t prescribe cannabis use for our patients, we don’t routinely tell them not to use it either, and perhaps we should.
Based on our study results, AYAs diagnosed with breast cancer may specifically want to avoid cannabis use altogether.
Accelerating the Development of Cancer
Another disturbing finding in your study was how quickly cancer risk rose following legalization of cannabis. Do you know why the drug accelerates the development of breast and testicular cancer so quickly?
We don’t have a definitive answer to this question since our study looked at epidemiological data at the population level. We know from the SEER and Canadian registries that cannabis use disorder is highest among 18- to 25-year-olds. The incidence pattern suggests that if cannabis use is accelerating the incidence of breast and testis cancers in young individuals, it is doing so in less than 10 years and probably less than 5 years.
Raising Awareness on the Benefits of Healthy Eating to Reduce Cancer Risk
Other cancer types are also rising in AYAs, including pancreatic, cervical, and colorectal cancers. Can cannabis use also be influencing the development of these cancers in younger adults?
While we didn’t see a pronounced association between cannabis use and the development of other cancer types, further studies are necessary. In this study, we focused on breast and testis cancers that appear most dramatically affected by the drug.
When we published our 2013 study in the Journal of the American Medical Association investigating the exponential incidence of metastatic breast cancer in young women, which had tripled in a 30-year period, we knew that the rise was too fast to be attributed to a genetic change, and hypothesized it had to be due to a lifestyle risk factor or carcinogen.6 Similarly, the recent increases in breast and testis cancers seem to be too rapid to be attributable to a genetic change.
New data are showing temporal correlations between lifestyle modifiable risk factors, such as the consumption of ultra-processed foods, and the risk of developing cancer and dying of the disease.7 The Centers for Disease Control and Prevention recently reported that children and adolescents, ages 1-18, consume 62% of their daily calories from ultraprocessed foods, while adults consume 53%.8
These data should be shared widely to raise public awareness that eating a more nutritious diet has the potential to reduce cancer risk, especially in adolescents and young adults.
DISCLOSURE: Dr. Johnson receives speaker fees from Servier Pharmaceuticals and Jazz Pharmaceuticals.
REFERENCES
1. Scott AR, Stoltzfus KS,Tchelebi LT, et al: Trends in cancer incidence in US adolescents and young adults, 1973-2015. JAMA Netw Open 3:e2027738, 2020.
2. Johnson RH, Speckhart A, Chien F, et al: Emerging evidence links cannabis use to increased risk of breast and testicular cancer in young Americans. Acad Oncol https://doi.org/10.20935/acadonco7758, 2025.
3. Reece AS, Bennett K, Hulse GK: Cannabis- and substance-related carcinogenesis in Europe: a lagged causal inferential panel regression study. JXenobiot 13(3):323-385, 2023.
4. Centers for Disease Control and Prevention: State Medical Cannabis Laws. February 16, 2024. Available at www.cdc.gov/cannabis/about/state-medical-cannabis-laws.html#:~:text=As%20of%20February%202024%2C%2047,of%20cannabis%20for%20medical%20purposes.
5. MallenbaumC: THC levels have quadrupled since the 1990s. Axios San Francisco, April 18. 2025. Available at www.axios.com/local/san-francisco/2025/04/18/cannabis-thc-high-levels-potency-chart.
6. Johnson RH, Chien Fl, Bleyer A: Incidence of breast cancer with distant involvement among women in the United States, 1976 to 2009. JAMA 309:800-805, 2013.
7. Chang K, Gunter MJ, Rauber F, et al: Ultra-processed food consumption, cancerrisk and cancer mortality: a large-scale prospective analysis within the UK biobank. EClinicalMedicine 56:101840, 2023.
8. Centers for Disease Control and Prevention: Ultra-processed Food Consumption in Youth and Adults: United States, August 2021-August 2023. Available at www.cdc.gov/nchs/data/databriefs/db536.pdf.
