Why can Naruto still use Rasenshuriken? When Naruto first used Rasenshuriken against Kakuzu the effect was massive. He destroyed   two of Kakuzu's hearts, but there were a huge after effect to him. If I'm not mistaken, Tsunade advised Kakashi to ask Naruto never to use the technique again due to the huge risk. So why is Naruto still able to use the technique and have no side effects after that battle? I may have missed out on something.
Naruto only used this jutsu one time in normal form (against Kakuzu, and got many side effects, like you said), the other times he was at Sage mode, at this state he didn't damage himself.

According to Naruto  Wiki:


  Naruto later improved the Rasenshuriken with Sage Mode. This allows
  him to throw it at his opponents, and removes the threat of damaging
  himself with the technique.