Don't say that. There are some minor negative side effects (e.g. weakened short-term memory) from the use of cannabis, and very (very very) small risks of some big side effects (prematurely triggering latent schizophrenia, for example).

It's the fact that it's harmless COMPARED TO MOST OTHER INTOXICANTS that really matters. Implying that cannabis is COMPLETELY harmless just gets everybody elses knickers in a twist. :)