Quick Answer:

If you get a sulfur or rotten-egg smell the first time a drain is used each morning, it usually means the water seal in the drain’s trap has dropped or organic buildup in the drain is releasing hydrogen sulfide gas. Start by running water to refill the trap, clean the visible drain parts and overflow channel, and check the garbage disposal if it’s a kitchen drain. If the smell returns quickly after those steps, the venting or a deeper buildup may need attention.

Why This Happens

Several common conditions create that morning sulfur smell:

• Trap seal loss: slow evaporation or a small leak lets the trap dry overnight, allowing sewer gas through.
• Biofilm and organic buildup: grease, soap, food and bacterial films on drain walls and in the disposal produce hydrogen sulfide gas when undisturbed.
• Overflow channel contamination: bathroom sinks have a hidden channel that traps grime and smells when not flushed.
• Vent problems: a blocked or poorly working vent can let gases accumulate and push odors back into the house when water first flows.

Step-by-Step What to Do

1. Refill the trap

Run cold water for 30–60 seconds in the affected drain first thing in the morning. This restores the water seal in the P-trap and is the quickest fix if the trap dried overnight.

2. Flush the overflow channel (bathroom sinks)

If the smell is from a bathroom sink, pour a cup of warm soapy water slowly into the overflow slot or use a narrow brush to scrub inside it. Flush by running water into the basin until the overflow clears any loosened residue.

3. Check and clean the garbage disposal (kitchen)

If this is a kitchen drain, run the disposal with a steady stream of cold water for 15–30 seconds. Inspect the splash guard (the rubber flange) and clean food and grease off it and visible parts of the grinding chamber. Grind a few ice cubes to knock debris loose, then run cold water to flush it out.

4. Clean biofilm around drain walls

Use a small brush or an old toothbrush and a paste of baking soda and water (or a gentle enzymatic drain cleaner per label instructions) to scrub the drain walls and the underside of the stopper. Flush with cold water. Regular cleaning removes the organic film that produces sulfur odors.

5. Verify vent draft if the odor returns quickly

If the smell comes back soon after refilling the trap and cleaning, the vent stack may be blocked. A simple sign is odors that appear with the first use rather than after standing water. Avoid attempting roof work yourself; instead, check for other signs like slow drainage or gurgling. If you need to describe the issue when you call a pro, mention whether the drain Drain Backs Up Only When Dishwasher Runs or the plumbing system makes noises like the Drain Burps Air After Water Shutoff.

What Not to Do

• Do not mix bleach and vinegar. That combination releases toxic chlorine gas.
• Don’t dump large amounts of boiling water into the drain if you’re unsure what the pipe material is—some older plastic pipes can soften or warp.
• Don’t use a flame or lighter near the drain to test for gas — that’s dangerous and unnecessary.

When to Call a Professional

• Smell persists after refilling traps and cleaning drains.
• Smell is present in several fixtures or throughout the house, suggesting a sewer line or vent stack problem.
• Persistent gurgling, slow drains, or sewage backup appear.
• You suspect a broken or leaking trap or damaged vent and are not comfortable inspecting on the roof.

Safety Notes

• Avoid mixing cleaners. Use one product at a time and follow label instructions.
• If you use chemical drain cleaners, ventilate the area and use gloves and eye protection. Do not use a plunger immediately after a chemical product has been applied.
• For roof vent checks and sewer-line work, hire a licensed plumber rather than attempting risky roof or excavation tasks yourself.

Common Homeowner Questions

• Why does it only happen in the morning?
  Traps can evaporate or settle overnight and microbial activity in stagnant drains produces noticeable gas after a period of inactivity.
• Will the smell go away by itself?
  Sometimes after running water and cleaning the drain the odor stops, but recurring smells usually need cleaning or venting fixes.
• Can I keep this from returning?
  Yes. Run water briefly before first use, clean drains and overflow channels regularly, remove food debris from disposals, and address any plumbing vent or trap leaks promptly.