Both locking and breaking syringes have some drawbacks. If you push the plunger down too soon, you have to get a whole new needle. Workers also can't easily adjust doses in them, nor mix medicines inside. To combat these problems, newer smart syringes allow workers to move the plunger around inside the barrel as many times as they want. After one injection, the nurse or another healthcare worker is supposed to set a lock that keeps someone else from re-using the needle. However, that means an unscrupulous worker could bypass the locking mechanism if he or she wants to use the syringe again. Poorly paid workers in developing countries sometimes save the needles they use at work and give out shots on their own to supplement their incomes, according to the WHO.