From Cannabis Culture:
Popeye is one of the world's most well-known and beloved animated characters. Since his creation, the pipe-puffing Popeye has become a global phenomenon, with millions of kids heartily munching on spinach in the hopes that it will make them as strong as the legendary sailor-man.
Yet is the spinach which gives Popeye his super-strength really a metaphor for another magical herb? Have children around the world been adoring a hero who is really a heavy consumer of the forbidden weed - marijuana?
The evidence is circumstantial, but it is there, and when added together it presents a compelling picture that, for many readers at least, Popeye's strength-giving spinach is meant as a clear metaphor for the miraculous powers of marijuana.