Careful disposal of used needles reduces the chances that the people in your community accidentally re-use or get poked by a used needle. This is a great example of harm reduction.
If you get your supplies from a free needle program, ask them if they accept full sharps containers. Many, though not all needle distribution sites, can take full containers back. Here in Winnipeg, Nine Circles Community Health Centre is one of the paces that will take your full containers and can give you new containers.
Street Connections is also another place that accepts full containers, plus they also have a map of needle drop-off locations in Manitoba. Just type in your location, the day of the week, and choose needle drop off, and they will generate a list of places where needle drop-off is available. Please note that these services are limited, and that “needle drop boxes” are designed to accept single needles, not full containers.