Let me begin with the obligatory disclosure that is required so that when you discuss a subject like this you're not just dismissed as a reefer addict hell-bent on the destruction of civil society:
I don't use drugs.
I have never used controlled substances outside of the prescription of a medical doctor.
I do not advocate drug use.
There that's out of the way.
I'm driving home and I hear a public service announcement begin on the radio. Allow me to paint with words the context of the commercial.
Young man: I tried marijuana once. Nothing bad happened.
Young woman: I tried marijuana once. Nothing bad happened.
Another young man: I tried marijuana once. Nothing... (sound clip of car skidding out of control interrupts dialogue)
The same young man can be heard in an emergency room asking frantically about the boy on the bicycle that he apparently hit and injured. He is unable to find out what's going on from hospital staff and he continues in a panic.
These types of commercials really get on my nerves. The guy was obviously high on marijuana and under the influence of this drug and he hits a kid on a bicycle and this is why marijuana is bad.
But this isn't the way our society understands and accepts intoxication of any kind. Alcohol is perfectly legal and incredibly intoxicating. It can impair a person to the degree that we have invented duties such as the "designated driver." Alcohol companies beseech their customers to "drink responsibly." This is evidence that as a society we can make the distinction between use and abuse. To abuse anything and put another person in harm's way is criminal. To enjoy a buzz in a bar or tavern and get a ride home is widely considered responsible. (I would personally argue that it still isn't responsible, but that's just me.)
According to these commercials we are to understand that the marijuana user is someone who will just throw caution to the wind and operate heavy machinery and drive cars and perform difficult surgeries while high because they have no ability to understand the difference between use and abuse. Apparently this level of discernment only exists for those intelligent alcohol drinkers and can't be left to marijuana smoking dolts.
We can debate about the health issues related to drug use, we can engage in intelligent discussions about how it impacts families, relationships, and whatnot. But don't play this stuff over the airwaves and expect intelligent people to believe it one bit. Don't piss down my back and tell me it's raining. My parents may have raised ugly children, but they didn't raise stupid ones.