A sink that drains slowly usually has buildup near the stopper, in the trap, or farther down the drain line. The most useful clue is whether the problem affects one fixture or several drains in the home.
Start with the stopper and accessible drain opening
Hair, soap, and debris collect just below many sink stoppers. Remove only components intended for routine household maintenance and clean visible buildup before reaching for chemicals.
A slow trap may hold debris
If the trap is accessible and you can protect the area, a local buildup may be removable mechanically. Do not take apart badly corroded piping or joints unless you are prepared to replace parts that may no longer reseal.
Several slow drains change the diagnosis
If a sink, shower, or toilet slows at the same time, the restriction may be in a shared line. Gurgling, water backups, or changing fixture levels are signs to stop treating each drain as an isolated problem.
Do not mix chemical drain cleaners
Different drain products can react and create splashes, heat, or dangerous fumes. If a chemical has already been used, tell anyone who may open the piping and follow the label before adding another treatment.