I inhaled Helium from just a few balloons, all for the fun of it...
A bit of research led me to discover that it can in fact be dangerous especially for someone with a lung or heart condition. Inhalation from a balloon is usually harmless, but don't try it if you know you have such a condition.
Inhalation from a pressurized tank is apparently more dangerous. If you feel dizzy and extremely unwell besides having your pitch altered, do seek medical assistance immediately.
Inhaling helium displaces oxygen, thus depriving your body of oxygen when the helium is in your system. Inhaling other inhalants does the same too!
Alright, I shan't get too Scientific... After all, we human beings just have a knack of risking our lives for fun =)
Have a blessed day!