In a recent double-blind, placebo-controlled study, using both smoked marijuana and THC infusions, Mathew et al studied the effect of tetrahydrocannabinol (THC) on blood pressure, pulse rate and blood flow to the brain to determine the extent of the phenomenon of dizziness or faiting associated with blood pressure drop when standing up after smoking a joint. A blood pressure drop that results in fainting ‘has considerable clinical relevance. In healthy individuals, it can cause injuries including lacerations and fractures. In individuals with pre-existing cerebrovascular disorders it can lead to stroke and sudden death’. Further it complicates a variety of diseases including multiple sclerosis, diabetes mellitus, Shy Drager syndrome, nephrosis, chronic fatigue syndrome, Parkinsonism, organic dementia and cervical myelopathy’. In ‘elderly ambulatory men’ it was found to be a significant and independent predictor of mortality. 28% of those studied reported severe symptoms, both with smoked marijuana and the THC infusions. The study found that the most marked autonomic change caused by marijuana was increased pulse rate. ‘ The results of the study clearly show loss of cerebral autoregulation and postural syncope (fainting when standing up) after ingestion of marijuana/THC. However the mechanism responsible for these phenomena is unclear.

Note: the 29 subjects were all experienced marijuana smokers. The study was reviewed and approved by the Institution Review Board (IRB) at Duke University Medical Center.
Source: A transcranial Doppler study of the hemodynamics. R J. Mathew et al Pharmacology:
Biochemistry and Behavior 75 (2003) 309-318