The concentration of THC, the active compound, is much higher than in the past. But since no one has ever overdosed on marijuana, it’s difficult to say why that matters. Yes, if someone has a history of mental illness, it’s not that smart to experiment with the cannabinoid receptors in the brain. But it isn’t smart for such people to take any drugs — or too much alcohol — for that matter. For most people, stronger pot merely translates into a need for less of it to get the same effect. Too much and you’ll likely nod off — and wake up later with no hangover. If pubs served pot rather than beer, crime rates would plummet.