Packaging says repeatedly not to throw leftover medication (in this case, colonoscopy “cleansing liquid”) down the drain. Ok. So I think pouring it into the yard is wrong, as is dumping it in the street, obviously. I really don’t want to keep it around until the next leftover drug collection by my municipality, so what the heck do you guys do? I guess I can’t recycle the big plastic jug unless I get rid of that stuff, so throw the whole thing in the trash bin? What am I missing here?
Well, you were supposed to drink all of it… which would result in it going down the drain. How could it possibly be a problem to flush it?
Yeah, but in my case, I wasn’t supposed to drink all of it, and I have no idea why. Instructions said mix 4 liters, drink 2. Plus we have a septic tank and god knows what might happen there. 
According to this - you can mix it with kitty litter or used coffee grounds and throw it away…
[ul]
[li]Mix all leftover liquid medication in a blender,[/li][li]add fruit,[/li][li]add alcohol,[/li][li]add juice,[/li][li]add more alcohol,[/li][li]add food coloring,[/li][li]add ice,[/li][li]serve with cute umbrella at your 4th of July party[/li][/ul]
stand back and watch the fireworks.
Do NOT attempt if you wish to stay out of jail.
Why not? Just flush it down the toilet and let it clean out your plumbing. 
What exactly is the medication? You mention “colonoscopy “cleansing liquid””, do you mean a colonoscopy prep medication like Gavilyte, MoviPrep, Golytely, Suprep, and the like? In that case, the medication is just large chain polyethylene glycol, with added electrolytes. I don’t see any negatives is just throwing the little amount left in your jug down the drain, even with a septic tank.
When you are looking at the negatives of throwing it away, make sure you don’t confuse it with antifreeze, Ethylene glycol and Propylene glycol, while the names are similar, the chain length in PEG (Polyethylene glycol) changes the biological effect… Drinking Ethylene glycol will kill someone, drinking polyethylene glycol will give someone the runs.
Yes, but isn’t that true for all medications?..
…but you’re not supposed to flush 'em.
Where I am, in the UK, one is supposed to return unused medications to a pharmacist. I suspect you’re in the US, but it might be worth checking if that’s an option there.
No. Most medications are changed by the process of going through your body.
…is there a medication that is somehow safe to ingest into a human body, but would somehow cause damage to standard plumbing? I can’t think of a way these two things could be true at the same time, although I can’t be totally, completely sure of this.
Note to self: However, do not mix into my coffee or give to cat.
Thanks to this BRAND new handy-dandy receptable, you no longer have to worry about your excreted medications contaminating the local water supply!
You take it to your local chemist / pharmacist for proper disposal.
It’s not, I suspect, the plumbing they’re worried about. The issue is that many drugs are not eliminated by standard processing of sewage and so we’re ending up with wildlife on Xanax and birth control. Which is not really as amusing as it sounds.
A lot of communities are having annual collections of unused pharmaceuticals. Or you could take it back to the pharmacy.