Is helium deadly

Yes you can, but only in vast amounts, 1 balloon isn't going to kill you, but it might make you a bit dizzy because helium speeds up the way oxygen leaves the body, so by taking too much helium, you can actually die from suffocation.

Be careful when using helium, because as funny as it can be to sound like Munchkins from the Wizard of Oz, it can be dangerous, and if over used like a drug, fatal.