It only needs a hechsher if it’s prepared. If you’re taking the raw plant, it doesn’t need a hechsher.
You can’t really overdose on marijuana. Can you have taken “too much”? Of course. It gets you high. Don’t drive, operate machinery, etc. Treat it as if you’ve been drinking.
Personally, I think it should be legal. It mostly has the same effects as alcohol. I think the government should legalize it and tax it heavily. That gets rid of the dealers, and generates income for the government.