Working out is good for you: this we all know. What you might not know is that immediately after exercising, pot smokers have higher THC levels in their blood: a "second hit" if you will.

A study published in Drug and Alcohol Dependence found that levels of THC were higher by an average of 15% immediately after users exercised. While the high is good, users need to know that the 15% could be enough to trigger a positive blood test when the results might otherwise be negative.

The study used 15 regular pot users who smoked about one joint a day. Blood was taken before and after a 35-minute exercise session on a stationary bike. All had not smoked 24 hours before the test, and body fat and BMI were also recorded.

The study found that the changes in THC (ranging from zero to 34%) increased with the amount of body fat the subject had.

The data may explain cases like that of a jockey who had to lose weight for a race and tested positive for cannabis long after he’d said he had last smoked it -- and instances of users testing positive in rehab when they exercise and lose weight despite being abstinent.

The rise was short-lived, lasting for less than two hours, but understanding how a “second hit” of THC from exercise -- and traumatic events -- could affect judgment and behavior will be critical to figuring out the role that THC testing can play in a variety of settings, from roadside stops of impaired drivers to autopsy reports to determine cause of death.