Tsunade Temporarily Treats Rasenshuriken Like a Forbidden Technique
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The Technique Works · Naruto's Own Arm Is the Problem · A Later Delivery Method Changes the Risk
Naruto's first completed Rasenshuriken is powerful enough to destroy Kakuzu's chakra network at a microscopic level, but the same close-range contact damages Naruto's arm. Tsunade warns him against using it again because continued exposure could permanently ruin his ability to mold chakra. Sage Mode later lets Naruto throw the technique instead of holding it through the impact.
What changed
The technique does not become safer because its destructive effect is reduced. Naruto changes how it is delivered so the part that made it unacceptable to his own body is no longer attached to him at detonation.