Inhaling helium is not funny or amusing.  It is incredibly dangerous.  For some it is deadly.

Last weekend in Eagle Point, Oregon Ashley Long went to a slumber party.  Instead of going to that party she went to another one.  Marijuana was there but isn't what killed her.  Taking hits off the helium tank did.

We've seen it in movies and many of us have inhaled helium from a balloon to pitch-up the voice for yucks.  We've graduated to the tanks used to fill the balloons and with the pressures involved the danger has gone up too.

Helium alone crowds out oxygen and is an effective agent in "suicide kits".  But under pressure helium can enter the bloodstream and cause an embolism.  This is similar to what can happen to divers who ascend too quickly.

For more see this source article.