Older cannabis literature paints a gentle picture of the heart and circulation: blood vessels widen, blood pressure falls, and rodent studies suggest protection against atherosclerosis and stroke damage. Every one of those findings is real. Since 2024 there has also been a body of human data pointing the other way, and it is larger: a US survey study of more than 430,000 adults (Jeffers et al., Journal of the American Heart Association, 2024) and a meta-analysis of 24 studies (Storck et al., Heart, 2025).
If you have diagnosed heart disease, a previous heart attack or stroke, an arrhythmia, or uncontrolled high blood pressure, discuss cannabis with a cardiologist before using it at all — including a prescribed product. This is not a formality. Nothing here is medical advice. In Thailand cannabis flower is a controlled herb dispensed only on prescription form PT 33 by an authorised practitioner.
1. What Cannabinoids Actually Do to the Circulation
The short-term effects are well described and pull in two directions at the same time:
- Heart rate rises. A fast heart rate (tachycardia) is one of the most consistent short-term effects of THC.
- Peripheral vascular resistance falls, and with it blood pressure. This is what causes light-headedness on standing (orthostatic hypotension).
In a healthy circulation, that combination is tolerable. In a heart with limited coronary reserve, it is exactly the wrong pairing: higher oxygen demand at lower perfusion pressure. This is how cannabis can bring on myocardial ischaemia.
Smoking adds a separate harm. Combustion produces carboxyhaemoglobin, which reduces the blood’s ability to carry oxygen. This is one reason smoking is the route medical-cannabis prescribers generally advise against, and smoked cannabis has been linked to a higher risk of heart attack and stroke.
2. The Claims in the Older Literature
Three claims appear again and again, and each deserves a specific answer.
“Cannabinoids may lower blood pressure by dilating blood vessels.” This is true in the short term. But cannabis can also raise heart rate at first, and with it blood pressure in some users. The honest version is: a short-lived effect on the circulation that can go both ways, not a treatment for high blood pressure. No cannabinoid is licensed for hypertension anywhere, and blood pressure is managed over decades, not minutes.
“Low-dose THC prevents atherosclerosis.” This refers to Steffens and colleagues (Nature, 2005). In mice bred to develop atherosclerosis, oral THC at 1 mg/kg/day slowed the progression of the disease. The effect worked through CB2 receptors on immune cells, with inflammation as the mechanism. The authors stressed that this dose was lower than the dose usually linked to psychoactive effects. It is a real and much-cited finding. It is also a mouse study, using a mouse dose that cannot be converted directly to human use, and the treatment has never been tested in people.
“Cannabis halves stroke damage.” This refers to work by Aidan Hampson and colleagues at the US National Institute of Mental Health. In 1998 (PNAS) they showed that THC and CBD act as antioxidants and protect cultured rat neurons against glutamate toxicity. A 2000 review by the same group (Annals of the New York Academy of Sciences) reported preliminary studies in a rat model of stroke, suggesting CBD might work as well in living animals as in the dish. We could not trace the widely repeated “halved” or “up to 50%” figure to either publication. Either way, the work is preclinical. As with the preclinical cancer literature, protecting brain tissue in a rodent model, with the drug given around the time of the injury, is a very long way from a treatment for human stroke.
None of these three is made up. But all three are being stretched to describe a human clinical benefit that no human study has shown.
3. What the Human Data Now Shows
This is the part that has changed most in recent years, and it changes the overall balance, not just a detail.
A large cross-sectional analysis of 434,104 US adults aged 18–74 (Jeffers et al., J Am Heart Assoc, 2024) used survey data collected between 2016 and 2020 (Behavioral Risk Factor Surveillance System). Compared with non-users, daily cannabis users had 25% higher odds of heart attack and 42% higher odds of stroke. Among younger adults at risk of early heart disease, the combined outcome of coronary heart disease, heart attack and stroke showed 36% higher odds regardless of tobacco use, and the link was stronger the more often people used cannabis.
A meta-analysis of 24 observational studies (Storck et al., Heart, 2025; 17 cross-sectional, 6 cohort, 1 case-control) found the following risks compared with no cannabis use:
| Outcome | Relative risk |
|---|---|
| Acute coronary syndromes | 1.29× |
| Stroke | 1.20× |
| Cardiovascular death | 2.10× |
Using only the cohort studies gave similar results.
4. How Much Weight That Deserves
Both bodies of evidence are observational, and the researchers themselves state that the limitations are substantial.
The survey study relied on self-reported cannabis use and self-reported heart conditions, which are open to recall bias. It had no baseline cholesterol or blood pressure measurements, and it looked at each participant only once. Its authors called for prospective cohort studies and stated that the findings show association, not causation.
The meta-analysis had a different problem: the pooled studies had no standard way of measuring cannabis exposure. It could not tell whether heart events happened during active use, could not separate smoked from eaten cannabis, and could not assess potency.
Robert Page II, PharmD, of the University of Colorado, who chaired the American Heart Association’s 2020 writing group on cannabis and heart health, commented on the 2025 meta-analysis to the cardiology news site TCTMD. His summary is a useful way to read observational data like this: “I don’t put a lot of faith in the magnitude, but I put a lot of faith in that there’s an issue here.”
That is the right balance. How big the risk is remains uncertain. That there is a cardiovascular warning signal is not. Note also the imbalance in the evidence: the protective claims rest on rodents, while the risk signal rests on hundreds of thousands of humans. The measurements are imperfect, but the subjects are human.
5. Route Matters More Here Than Almost Anywhere
Because carboxyhaemoglobin and combustion products are part of the mechanism, the way cannabis is taken is not a minor detail for this group.
Smoking is the worst option for anyone with cardiovascular risk. Vaporisation removes combustion products, but it does not remove the effects on the circulation (heart rate up, blood pressure down on standing), because those come from THC itself.
The pooled data above could not separate smoked from eaten cannabis. Our own reading, which is not a finding of the meta-analysis, is that the estimates probably reflect mostly the most common route, smoking. The data cannot confirm this, and the risks cannot be assumed to be the same for every route.
6. Who Should Be Most Careful
Putting the mechanisms and the data above together:
- Known coronary artery disease, previous heart attack or stroke, or an arrhythmia — the strongest caution. Ask a cardiologist whether cannabis is appropriate at all.
- Uncontrolled high blood pressure.
- Older patients, who face low blood pressure on standing and the risk of falls on top of any heart risk.
- Anyone on heart medication — cannabinoids are broken down by cytochrome P450 enzymes that also process many heart drugs.
- Anyone about to have surgery. Current US consensus guidelines (ASRA Pain Medicine, Shah et al., 2023) recommend that every patient be asked about cannabis use before an anaesthetic. Tell your anaesthetist about any cannabis use, and follow your hospital’s pre-operative instructions. This is one of the situations where hiding it can have immediate consequences.
7. The Honest Balance
There are real research questions about the anti-inflammatory effects of CB2 in atherosclerosis, and about whether cannabinoids protect the brain after a stroke. They are worth pursuing, and researchers are pursuing them.
There is now also a consistent observational signal that cannabis use is linked to more heart attacks, more strokes and more cardiovascular deaths. How big the risk is remains uncertain, and causation is not proven.
For a patient today, the second point should carry more weight. A mechanism seen in animals does not cancel out a risk signal seen in humans, and the burden of proof lies with whoever claims a therapeutic benefit.
8. In Thailand
Cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568, in force since 26 June 2025. Flower may be dispensed only against a prescription on form PT 33 from an authorised practitioner. Each prescription covers up to 30 days, is single use (no refills), and the flower must come from a GACP-certified cultivation site. We are not aware of any national guideline that lists heart disease as a reason to prescribe cannabis. A prescriber assessing a patient with a heart history would be weighing the risks above, not expecting any benefit to the heart.
If you have a heart condition and have raised cannabis with a cardiologist in Thailand or the region, what did they say, and did they distinguish between routes? The evidence suggests that smoked and non-smoked cannabis may not carry the same risk, and it is worth knowing how often doctors actually make that distinction in the consulting room.
This article is part of the Asiannabis Community educational series on the science of medical cannabis. It is general information, not medical advice. If you have heart disease, speak to a cardiologist before using any cannabis product.