The popular method is baking soda, then vinegar, then hot water, and the science behind it is real but modest. Baking soda is a base and vinegar is an acid. Combine them and you get a fizzing reaction that releases carbon dioxide. That foaming action can agitate loose gunk on the pipe walls, and the mild alkalinity of baking soda helps cut light grease and neutralize odors. It's gentle cleaning chemistry, not a blockage-clearing tool.
Where this method actually earns its keep is maintenance and smell. A kitchen drain that's started to stink, or a bathroom sink draining a touch slow from soap and toothpaste film, often responds well. The baking soda deodorizes, the fizz lifts surface residue, and a hot-water flush carries it down the line. Used every few weeks, it's a reasonable way to keep a healthy drain healthy and odor-free.
The limits are where people get frustrated. If your drain is truly clogged, fully stopped or backing up, baking soda and vinegar won't save you. The reaction is weak and short-lived, and most of the fizz spends itself in the trap before it ever reaches the real blockage. It does almost nothing against a wad of hair, a solidified grease plug deep in the line, or a foreign object, which are the causes behind most serious clogs.
Even so, it's the right first move precisely because it's safe. Caustic chemical drain cleaners run on lye or sulfuric acid that throw off heat as they work. That heat can soften PVC, corrode older metal pipes, and chew up rubber gaskets, and if the cleaner just sits on a stubborn clog without clearing it, now you've got a pipe full of dangerous chemical that can splash back when a plumber opens it. Baking soda and vinegar carry none of that risk.
To do it right, clear standing water from the sink first so the mixture reaches the pipe walls. Pour about a half cup of baking soda down the drain, follow with a half cup of white vinegar, and cover the opening to push the reaction downward. Let it sit 10 to 15 minutes, then flush with a kettle of hot, not boiling, water. Repeat once if you saw improvement. If nothing changes after two tries, the method isn't the answer.
Knowing when to stop matters. If the drain is still slow or stopped after a couple of attempts, the next step is mechanical: a plunger, then a drain snake to physically reach and break the clog. A clog that keeps returning, affects more than one fixture, or comes with gurgling and bad odors usually points to a deeper problem in the branch line or main, and that's a call for a plumber rather than another round of pantry chemistry.