A bathroom sink that is not draining is usually caused by a clog in or near the drain—most often hair, soap scum, toothpaste buildup, or debris caught in the P‑trap under the sink.

Common causes

  • Hair and gunk in stopper : Hair binds with soap and toothpaste around the pop‑up stopper and just below it, quickly slowing or stopping drainage.
  • Soap scum and toothpaste: These leave a sticky film inside the drain and pipes that narrows the passage over time.
  • Clogged P‑trap: The U‑shaped pipe under the sink traps debris like hair, sludge, and small objects and is one of the most frequent blockage points.
  • Older or corroded pipes: Rough, corroded pipe interiors catch more debris and can make a simple clog more likely and harder to clear.
  • Vent or deeper line issues: A blocked vent or main drain problem can make multiple fixtures drain slowly, including your bathroom sink.

Safe DIY checks and fixes

  • Remove and clean the stopper: Lift out or unscrew the stopper, clean off hair and sludge, then run hot water to see if flow improves.
  • Try a plunger (gently): Cover the overflow hole with a wet cloth, fill the sink a bit, and plunge with short, firm strokes to move a soft clog.
  • Clean the P‑trap: Place a bucket under the trap, loosen the slip nuts by hand or with pliers, remove the trap, and rinse out debris before reinstalling and checking for leaks.
  • Avoid harsh chemical cleaners: Many plumbers recommend against frequent use of strong drain chemicals because they can damage pipes and do little for solid hair clogs.

When to call a plumber

  • Water backs up in multiple fixtures (toilet, tub, other sinks), suggesting a main line or vent issue rather than just the basin.
  • You see signs of leaks under the sink, sewage smell, or standing water that won’t go down even after basic cleaning of the stopper and P‑trap.
  • The plumbing is old or corroded and you repeatedly get slow drains even after cleaning, which may indicate damaged pipes or poor installation.

Quick “forum-style” note

A lot of recent home‑repair forum threads describe people fixing a “sink not draining bathroom” issue simply by pulling the stopper, fishing out a wad of hair, and rinsing the P‑trap—usually a 20–30 minute job if the trap is accessible and the nuts aren’t seized.

If unsure, or if you are uncomfortable taking pipes apart, stop at basic cleaning of the stopper and contact a licensed plumber to avoid damage or leaks.

Information gathered from public forums or data available on the internet and portrayed here.