I used to smoke shitloads, and was something of a conesseur of strains of weed...
I know that one single drag can have far greater effects than a pint of lager, and you're more likely to THINK you're ok when you're stoned - plus certain strains may only hit you with the full effects a while after you've smoked it.
Granted, the slap-down stone of Black Domina wont have you jumping up to drive, but the light, almost amphetamine high of Kali Mist or White Widow may well do... but that light stimulant high gives way to a heavy stone or even hallucinations.
I'm not against ganja in the slightest, but I went from smoking pure skunk every single day to nothing when I started riding bikes.
And these were my reasons for doing so.
