Plumbers Say To Stop Pouring These 9 Liquids Down The Drain
It’s easy to assume that if something is liquid, the sink can handle it. It disappears fast, makes no noise, and gives no warning. That illusion is what causes long-term plumbing damage.
After replacing drain assemblies and seeing what builds up inside pipes, I stopped thinking of the sink as a disposal system. Plumbers repeat the same message: certain liquids do not belong in your drain, even if they seem harmless at first.
Here are nine that cause the most damage.
1. Fats, Oils, And Grease
Cooking oil looks harmless when it’s hot and fluid. Once inside cooler pipes, it solidifies and sticks to the walls. Over time, it traps food particles and reduces pipe diameter.
This buildup does not stay contained to your home. Grease combines with debris in sewer systems and creates major blockages.
Let it cool, wipe it out with paper towels, and throw it in the trash.
2. Leftover Frying Oil From Pans
This is slightly different from grease drippings. Many people pour used frying oil down the drain after cooking because it’s still liquid. Once it cools inside plumbing, it behaves the same way as grease.
Even small amounts poured repeatedly create layers.
If you cook often, this becomes a slow, predictable clog.
3. Coffee Grounds Mixed With Water
Coffee grounds feel harmless because they are small. When rinsed down with water, they form a dense sludge that settles in bends and traps debris.
Unlike food scraps, grounds do not break down easily. They accumulate and restrict flow.
It is better to toss them in the trash or compost.
4. Starchy Pasta Or Rice Water
The cloudy water left after boiling pasta or rice contains starch that thickens as it cools. Inside pipes, that starch can coat surfaces and contribute to buildup.
If poured down the drain occasionally, it is unlikely to cause immediate damage. When combined with grease or other residue, it helps create sticky layers inside plumbing.
Let it cool and dispose of it gradually with plenty of cold water, not all at once into a slow drain.
5. Paint
Even water-based paint contains binders and additives that should not enter plumbing. It can cling to pipe walls and dry in layers, restricting flow.
In many areas, pouring paint down the drain is illegal because of environmental contamination risks.
Unused paint belongs at a hazardous waste facility.
6. Motor Oil And Automotive Fluids
Motor oil contains heavy metals and toxic compounds. A small amount can contaminate large volumes of water.
It is illegal in most regions to pour automotive fluids into drains or trash. They require proper recycling.
This is about more than plumbing damage. It affects groundwater.
7. Medications
Flushing medication down the sink does not clog pipes, but it introduces pharmaceutical compounds into wastewater systems. Treatment plants are not designed to fully remove these substances.
That contamination can enter rivers and drinking water supplies.
Use pharmacy take-back programs whenever possible.
8. Boiling Water Poured Directly Into A Clog
Boiling water is often suggested as a quick fix. If there is already a blockage, the water can sit in one section of pipe and expose joints or PVC to extreme heat.
Repeated stress weakens seals and fittings.
Hot tap water is safer. Boiling water is not a universal solution.
9. Liquid Drain Cleaners
Liquid drain cleaners rely on chemical reactions that generate heat and use caustic compounds to break down clogs. Those same reactions can damage metal and plastic pipes.
Repeated use weakens pipe walls and increases the risk of leaks. If the cleaner fails to clear the clog, the corrosive liquid remains in your system.
Mechanical cleaning methods are safer long term.
Why This Matters
Most plumbing damage builds slowly. There is no dramatic failure at first. Pipes narrow. Seals weaken. Flow slows down. Then one day, the system backs up.
The sink feels like the easiest place to get rid of unwanted liquids because they disappear from sight. That disappearance is misleading.
If you want your plumbing to last, the rule is simple: if a substance thickens, hardens, corrodes, or contaminates, it does not belong in your drain.