Australian researcher Nadia Solowij has demonstrated that marijuana does change your brain, albeit quite subtly. The effect, however, is permanent and the more you take marijuana, the worse it gets.

Using a test which asks people to hit a button when they hear a particular tone of specific pitch, length and location (left or right ear), marijuana takers make more mistakes, even when not under the drug’s influence at the time. Brainwave measurements during the tests suggest that they are less able to filter out distracting information.

Ed.- Nadia’s book Cannabis and Cognitive Functioning (Cambridge University Press - ISBN 0521591147) describes her work but also critically reviews previous studies. The New Scientist described it as an essential read for anyone with a serious interest in the marijuana debate. Price £50 (or free if requested through your local library)