In the very short term, cannabis elevates blood pressure and heart rate. However, prolonged administration of THC, in both humans and experimental animals, results in the lowering of both blood pressure and heart rate. Animal studies suggest that this may be due to a reduction in adrenergic tone, which is also how beta-blockers, such as metoprolol, work as anti-hypertensives.
However, cannabis will not lower blood pressure or heart rate excessively.
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