When you take out-dated medicines to your chemist to 'waste', he/she puts them in a special container that can only be disposed of according the local laws, at least they do here in the US. They do NOT toss them down the drain. They can get heavily fined for doing that.

When I worked for a group of doctors in Marketing, they encouraged all employees to bring in unused medications and expired meds so they could be disposed of them in the same manner. It's a red plastic container that says 'Medical Waste' and all sorts of things go in there, including medications. Sharps go into special Sharps containers. A contractor comes and hauls them away.

By all means, take medicines to your chemist for proper disposal.