Say it Ain't so Snopes:

As a hard-nosed, but reasonable skeptic, I love Snopes.com. So many urban myths, so little time. I generally appreciated learning all the things they debunked that I believed as a teenager. But with the death of Mary Travers I checked up the "Puff The Magic Dragon" tale, one I was sure was true, and alas found out that it was an urban myth. It turns out Robert De Niro's character on "Meet the Parents" was right.

But then again, John Lennon insisted that "Lucy in the Sky with Diamonds" was NOT about LSD and I for one don't believe that for a second.