Nobody can give you a clear answer, because the few studies disagree. In 72 young men who tested positive for THC at a routine medical examination, resting RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD was higher than in 72 matched non-users, 56.2 ms against 48.6 ms (Schmid and colleagues, 2010). In 12 men with chronic nerve pain, a measured dose of THC lowered the LF/HF ratio LF/HF ratio LF power divided by HF power, with no unit. It was proposed as a gauge of balance between the sympathetic and parasympathetic systems, but research shows it cannot measure that reliably. More about LF/HF ratio , a shift toward the vagal side (Weizman and colleagues, 2024). Older lab work in healthy volunteers found the opposite, and the one steady finding is that THC speeds the heart up soon after use.
What the studies found
Everyday users. The German study screened young men at a routine medical check. Seventy-two had THC in their urine and no other drugs; 72 others, matched for age and body mass, used nothing. The cannabis group had higher resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate variability (HRV): median RMSSD 56.2 ms against 48.6 ms, and a lower LF/HF ratio (1.62 against 1.87). Their scores on a well-being questionnaire were lower. This is a snapshot of two groups, so it cannot say what cannabis did to any one person’s HRV, and a positive urine test does not say when they last used it.
A controlled dose. The Israeli trial gave 12 men with chronic radicular pain either 0.2 mg/kg of THC under the tongue or a placebo, on separate days, and measured HRV and pain modulation before and after. THC lowered the LF/HF ratio compared with placebo and improved the body’s own pain-damping response. Brain scans linked the HRV shift to stronger connections between a brainstem control centre and the prefrontal cortex. The authors note in their introduction that earlier studies in healthy volunteers found THC raised sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system activity and lowered vagal activity Vagal tone The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power. More about Vagal tone , while a study of regular users found the reverse. Their result is for one dose, in a dozen men with chronic pain.
Heart rate. THC raises heart rate soon after use, and tolerance to this effect appears quickly with repeated exposure; in regular users the heart slows and the balance tips toward the parasympathetic Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system side (Jones, 2002). A faster heart leaves less room for beat-to-beat variation, which tends to pull HRV down for that window regardless of what the nervous system is doing, as the page on HRV vs resting heart rate explains.
No study has followed overnight wearable HRV around cannabis use, so there is no answer for the number most people are looking at.
How to read your own data
- Separate the smoke from the drug. Cannabis rolled with tobacco carries nicotine’s dose-related drop in HRV (Murgia and colleagues, 2019). See does nicotine or vaping lower HRV?
- Tag use and timing. Use in the evening may show in the first hours of sleep through a faster heart; tag it and compare those nights with your usual range using the method in building your HRV baseline.
- Do not read a higher number as approval. The large study found lower well-being in users even with higher HRV. The number is not a health verdict.
- Watch for companions. Late nights, alcohol and snacks often go with cannabis use and each moves the night’s number.
When to talk to a doctor
Talk to a doctor if cannabis use comes with palpitations, chest pain, fainting or anxiety, or if you are finding it hard to cut down. People with heart conditions should ask about THC’s effect on heart rate before using it. Cannabis is illegal in many places; this page describes research, not advice. HRV cannot diagnose anything.