Quick Answer:

If a drain runs fine at first and then suddenly slows or stops while you’re using it, the most common causes are a shifting clog, the stopper moving, a trap filling with debris, or a venting issue that allows the drain to gulp air and lose flow. Start with a few quick checks you can do yourself to narrow the cause: time how long until the slowdown, check the stopper, inspect the trap, listen for unusual sounds, and compare how the drain performs when you pour from a bucket versus letting the faucet trickle.

Why This Happens

• A small plug of hair, grease, soap scum or food can hold while flow is high and then shift to block the pipe when the flow reduces or changes direction.
• Pop-up stoppers or lift stoppers can move mid-use and block the outlet partially or completely.
• Air flow problems (poor venting) let the drain slip air in and out — you may hear gulping noises as water tries to pass.
• Less commonly, a trap packed with solids or an issue in the main line that only shows under certain flow conditions is responsible.
• If smells or residue are a recurring part of the problem, see related topics like Drain Is Slow Only After Using Soap for cases that follow soap and grease build-up.

Step-by-Step What to Do

Step 1 — Time how long until slowdown

Start the water and watch. Note the time from when the drain gets good flow to when it slows or stops. A few seconds to a minute suggests a shifting obstruction near the fixture; several minutes points to a trap or deeper partial blockage.

Step 2 — Check the stopper position

Make sure the stopper is fully open. Move it up and down by hand while running water to see if its position is causing the sudden stop. Remove the stopper if possible and test the drain without it.

Step 3 — Inspect the trap for debris

Place a bucket under the P-trap, loosen the slip nuts, and remove the trap to look for hair, grease or food that may shift under flow. Clean the trap and reinstall it carefully.

Step 4 — Listen for air gulping

Run the water and pay attention to gurgling or gulping sounds. That noise suggests a venting problem or an air pocket. If you hear this along with sudden slowdowns, the issue may be beyond the trap.

Step 5 — Test with a bucket pour vs faucet trickle

Pour a bucket (several gallons) of water straight into the drain and then try a slow faucet trickle. If the bucket pour goes through but the trickle stops, the problem is likely a partial clog or poor venting that can’t pass low flow. The bucket test helps tell the difference between a restriction that only stops low flows and a complete blockage.

Step 6 — Check if other drains are affected

Run nearby sinks, tubs, or floor drains. If more than one fixture slows or gurgles, the problem may be in a shared line or the main stack and will usually need professional help.

Step 7 — Try a basic clear and retest

• Use a cup plunger on sinks and tubs to try to dislodge the clog after sealing the overflow or adjacent openings.
• If a plunger doesn’t work, a hand-held drain snake into the trap or just beyond may help — stop if you can’t control it or feel resistance you can’t manage.

What Not to Do

• Do not keep running water to “push it through.” That risks an unexpected overflow or backing up into other fixtures.
• Avoid forcing a long auger or snake if you can’t control it; you can damage pipes or push the clog farther in.
• Don’t pour large amounts of strong chemical drain cleaners into a partially blocked drain — they can harm pipes and create hazardous conditions if you later need to work on the trap.

When to Call a Professional

• The slowdown happens in multiple fixtures, or you notice sewage smell or backups — this suggests a main-line issue.
• You hear persistent gurgling from vents, or the problem returns after you clear the trap.
• There’s visible damage, leaking joints, or you suspect a collapsed or broken pipe. If you can’t locate or clear the obstruction safely, call a plumber.

Safety Notes

• Wear gloves and eye protection when working under sinks; traps contain dirty water.
• Turn off water to the fixture if it begins to overflow or back up.
• Use hand tools you can control. If a tool slips or binds, stop and reassess rather than forcing it.
• Avoid mixing chemical drain products — they can react dangerously and make repairs harder for a professional.

Common Homeowner Questions

• Why does it sometimes clear after a bucket pour? A fast, high-volume pour can force a partial clog to move, showing the blockage is near the fixture and not full pipe failure.
• Could the pipe be collapsing? Collapse is rare in most modern plumbing; older clay or cast-iron systems can fail, but shifting clogs or vent issues are much more common.
• What if there’s a bad smell with the slowed drain? Persistent odors plus slow drains may mean trapped debris or sewer gas access; investigate trap seals and consider related issues such as Drain Smells Like Rotten Eggs in the Morning.