An extensive 2013 review of 66 studies regarding crash risk and drug use found that cannabis was associated with minor, but not statistically significant increased odds of injury or fatal accident.In the largest and most precisely controlled study of its kind carried out by the U.S. Department of Transportations National Highway Traffic Safety Administration, it was found that other "studies that measure the presence of THC in the drivers blood or oral fluid, rather than relying on self-report tend to have much lower (or no) elevated crash risk estimates. Likewise better controlled studies have found lower (or no) elevated crash risk estimates". The study found that "after adjusting for age, gender, race and alcohol use, drivers who tested positive for marijuana  were no more likely to crash than those who had not used any drugs or alcohol prior to driving".A 2018 study indicated that the number of fatal crashes involving marijuana after the recreational marijuana legalization or decriminalization increased in Colorado, Washington, and Massachusetts. Cardiovascular effects
Short-term (one to two hours) effects on the cardiovascular system can include increased heart rate, dilation of blood vessels, and fluctuations in blood pressure. There are medical reports of occasional heart attacks or myocardial infarction, stroke and other cardiovascular side effects. Marijuanas cardiovascular effects are not associated with serious health problems for most young, healthy users.
Researchers reported in the International Journal of Cardiology, "Marijuana use by older people, particularly those with some degree of coronary artery or cerebrovascular disease, poses greater risks due to the resulting increase in catecholamines, cardiac workload, and carboxyhemoglobin levels, and concurrent episodes of profound postural hypotension. Indeed, marijuana may be a much more common cause of myocardial infarction than is generally recognized. In day-to-day practice, a history of marijuana use is often not sought by many practitioners, and even when sought, the patients response is not always truthful".A 2013 analysis of 3,886 myocardial infarction survivors over an 18-year period showed "no statistically significant association between marijuana use and mortality".A 2008 study by the National Institutes of Health Biomedical Research Centre in Baltimore found that heavy, chronic smoking of marijuana (138 joints per week) changed blood proteins associated with heart disease and stroke.A 2000 study by researchers at Bostons Beth Israel Deaconess Medical Center, Massachusetts General Hospital and Harvard School of Public Health found that a middle age persons risk of heart attack rises nearly fivefold in the first hour after smoking marijuana, "roughly the same risk seen within an hour of sexual activity".Cannabis arteritis is a very rare peripheral vascular disease similar to Buergers disease. There were about 50 confirmed cases from 1960 to 2008, all of which occurred in Europe. Combination with other drugs
A confounding factor in cannabis research is the prevalent usage of other recreational drugs, especially alcohol and nicotine.