We've all heard our parents say it*: "Back in my day, dope was much better than it is now. It wasn't nearly as strong as what you kids smoke today."
Like much of the advice our parents give us (like always take out your contacts before you go to bed), this one is also true. The THC (tetrahydrocannabinol - the primary psychoactive compound in cannabis) concentration in marijuana has increased by as much as 12% over the last 30 years. This rise in THC levels is related to increases in the subjective 'high' feelings associated with smoking cannabis, like changes in perceptual sensations, contentedness, and increased appetite. However, THC is also linked to many of the negative consequences of cannabis use, including risk for dependence, attentional bias or distraction, impaired memory and cognition, and the potential emergence of psychotic symptoms.