KEY POINTS:
Marijuana potency increased last year to its highest in more than 30 years, says a report released by the White House.
University of Mississippi scientists found that the average amount of the active ingredient, THC, reached 9.6 per cent in 2007, against 8.75 per cent in 2006.
The 9.6 per cent level represents more than a doubling of potency since 1983, when it averaged just under 4 per cent.
