We did an access to information request with CSC some years ago, and they showed I think it was over 100 accidental needle stick injuries in that period of about five years, and not a single case where a needle was used as a weapon.
I can speculate about the reason why. It's because these are very valuable. They're so scarce. People don't want to use them as weapons and give them up because you have to pay to use them. In the jurisdictions and other countries where these needle syringe programs exist, they have not been used as weapons.