A bathroom sink that drains slowly is frustrating; a toilet that gurgles while the tub backs up is a different kind of signal. Pay attention to which fixtures are affected and whether the problem changes after water runs elsewhere in the home. That pattern is often more useful than trying another product under the sink.
Separate a local clog
If only one sink is slow, remove visible debris from the stopper and clean it, taking care not to push material farther into the drain. For a simple basin clog, a plunger made for sinks may help; cover the overflow opening with a damp cloth to improve the seal. Stop if water begins leaking beneath the sink or plunging makes the blockage worse.
Notice whole-home warning signs
More than one backed-up fixture, sewage odors, or bubbling and gurgling from drains can point to a blockage beyond a single fixture. Avoid running more water while you assess what is happening, especially if water is rising in a tub or floor drain. A licensed plumber can determine whether the issue is in a branch drain or the main line.
Skip repeated chemical attempts
Chemical drain cleaners can create a hazard for anyone who later opens or works on the pipe, and repeated use is not a dependable way to find the cause. If you have already used a product, tell the plumber what it was and when you used it. When several fixtures are affected or a backup returns, stop experimenting and arrange professional help.