In general, it's better to avoid the habit of flushing medicines or many household chemicals, like cleaners and disinfectants, down the toilet or drain. Obviously, there are case-by-case exceptions where it may be all right. But disposing in the trash is preferable since a well designed landfill will minimize the chance of the drugs leaching into the water supply or affecting wildlife or aquatic animals.

We can already detect a whole range of pharmaceuticals in our tap water and no one knows what long term effect they may have on us. We should do what we can to minimize adding to the contamination by not flushing more chemicals down the drain.

Edit: Like others, I was thinking KI pills, not water purification tablets. Hmm, I think I would still advise tossing them in the trash than down the toilet, though, although there is probably little harm.