Ok, this is from last year but some may find it interesting:
"Smoking marijuana can affect blood flow in the brain so much that it takes over a month to return to normal. And for heavy smokers, the effects could last much longer, a new study suggests.
Regular marijuana use can harm memory and the ability to make decisions, according to Jean Cadet at the National Institute on Drug Abuse in Baltimore, Maryland, US. To find out why, he monitored the flow of blood through the brains of 54 marijuana smokers, among whom the heaviest user smoked 50 joints every day.
People who smoked cannabis had higher blood flow through their brains than non-users. Yet there was also greater resistance to the blood flow, suggesting that cannabis changes the blood vessels in the brain in a way which hinders oxygen in reaching the tissue effectively. In an attempt to compensate, extra blood is sent to that part of the brain, increasing resistance but probably failing to get enough oxygen through the vessels, Cadet suggests.
Cadet and his colleagues used an extremely sensitive non-invasive technique called transcranial Doppler sonography to "see" the blood flow through individual arteries from the head's surface.
After a month without cannabis - during which the volunteers agreed to remain in a clinic, with no access to marijuana - Cadet repeated the sonography. The resistance to blood flow of light and moderate users - who usually smoked an average of 11 and 44 joints per week, respectively - was starting to return to normal.
But there was no improvement observed in the heavy users, who smoked an average of 131 joints per week. "We were surprised because we'd expected that as marijuana cleared the system things would improve," says Cadet. He now wants to see if there is a link between the changes in the brain's blood flow and the extent of neuropsychological problems.
To eliminate the effect of tobacco in the joints, Cadet compared his results to those obtained from smokers, who showed normal blood flow. But, says William Notcutt of James Paget Hospital in Norfolk, UK, the longer-term effect on the brain may not have been caused by the same substance that produces the high.
"Somebody smoking 50 joints per day is getting a huge number of carcinogens from the marijuana plant," he notes. "We know the cardiovascular effect [of cannabis] is very complex and multi-factorial so it's not as simple as with other drugs. The group that needs to be studied now is people that use high quality medicinal extracts."
He adds that the results may also be different for people that only smoke marijuana occasionally, and so are exposed to lower doses of the toxic substances."