Marijuana has provable health risks (so does every herb and drug on every market). The risks include;

Lung damage (you cannot smoke ANYTHING without inhaling carcinogens)
Impaired judgement
Short term memory loss
Paranoia
Heavy metal (nickel) poisoning in patients with depression

That said, the BENEFITS of Cannabis far outweigh the majority of associated risks. Also it is significantly less harmful than nicotine and alcohol, even when used recreationally.