I have disposed of old expired medication down the toilet before when I had no other option but since my Pharmacy has started taking back old expired medication to dispose of I no longer dispose of the medication in the toilet.
When you dispose of your old expired medication in the toilet it can contaminate the water supply when you flush the water down the toilet.
If you're on city sewer the toilet water and medication will eventually reach a sewage treatment plant where the water is disinfected and then sent back to the water supply or ground water.
So although the city sewer treatment plant is supposed to disinfect the water the medication residue etc could still be partly in the water.
If your pharmacy or some other place will take back old medication for disposing of then I would do that instead but if you have no other option then flushing the medication down the toilet is not a really bad thing to do but if you have other options it would be best.