There are two reasons to keep an eye on what goes down your kitchen sink drains. For one, you don't want to put anything down there that clogs up the pipes and requires a plumber to come in and make repairs. For two, what you put down the sink eventually ends up in the sewers, so if you are eco-minded you want to be careful about dumping just anything. That said, there are a number of items one should definitely not slip down the drain, even if you are confident in your heavy-duty garbage disposal.
These need to go in the trash or else disposed in a safer manner:
1) Cleaning products - Check the labels of any expired cleaning products. If you see any warnings that announce the liquids, gels, or powders are toxic or corrosive, are flammable and cause harm if swallowed, do not pour down a drain! Corrosive products could prove harmful for your plumbing system, and residue left in the sink could cause illness in your home - especially if somebody accidentally drops a cup, then uses it without thoroughly cleaning it.
2) Pesticides - Anything that is categorized as a poison should not go down your sink. This includes insect killers, rat poison pellets... anything you use to get ride of unwanted critters and weeds. This stuff gets into the systems and causes environmental harm, and you definitely don't want poison residue in your sink.
3) Paint - Paint and products like thinners and similar oil-based products need to be disposed of differently. Paint is an obvious no-no, as it can stain your sink and the insides of your pipes, promoting quick corrosion and damage. If you're not sure how to dispose of unused paint, consult with a home improvement specialist.
4) Medicine - Cough syrups, pills, and other types of meds should not be flushed down the sink.
Bottom line: if you are not certain that the product is biodegradable, do not put it down the drain. Contact the city if necessary with regards to finding a public collection center for hazardous waste to properly dispose of anything that could be poisonous or damaging to your home.
Kitchen sinks are used mainly for the preparation of food. Would you plant an open paint can or jug of insect poison on your dinner table? Of course not! So why put them in an area where you prepare food for your family? Use your common sense to keep your kitchen safe.