Here is what likely happened in detail:
1. Hydrogen peroxide entered the drain and started breaking down
Hydrogen peroxide (H₂O₂) naturally decomposes into:
2H₂O₂ → 2H₂O + O₂ (oxygen gas)
This breakdown happens faster when hydrogen peroxide contacts:
- bacteria and organic matter
- metal surfaces
- dirt and residue
- enzymes and minerals
The oxygen bubbles produced can loosen some grime and disturb buildup.
2. It attacked years of organic buildup
After 8 years, a shower drain commonly contains:
- Hair strands
- Soap residue
- Body oils
- Dead skin cells
- Shampoo/conditioner deposits
- Bacterial slime (biofilm)
Hydrogen peroxide can oxidize some of these materials. You may have noticed:
- fizzing or bubbling
- foam rising
- a crackling or popping sound
- a temporary “clean” smell
The bubbles were oxygen escaping as the peroxide reacted with organic material.
3. The bubbling may have pushed debris loose
If the drain had a layer of slime inside the pipe, the oxygen bubbles could have:
- lifted small pieces of residue
- loosened hair caught near the drain opening
- disturbed bacteria colonies
- moved debris deeper into the pipe
Sometimes this improves drainage temporarily. Sometimes it only moves the blockage farther down.
4. A smell may have appeared
Possible smells include:
Fresh/chemical smell
- leftover peroxide or oxygen release
Musty smell
- disturbed biofilm or old organic buildup
Rotten smell
- trapped organic material being disturbed, especially if the drain trap contained stagnant water
The smell usually fades after flushing with water.
5. It may have affected the drain bacteria
Hydrogen peroxide is an oxidizer, so it can kill or reduce many surface bacteria. In a shower drain, it may temporarily reduce:
- odor-causing bacteria
- slime-forming microbes
However, it does not sterilize the entire plumbing system because:
- it becomes diluted quickly
- it decomposes
- deeper pipe biofilms remain
6. If your drain was slow, the result depends on the blockage type
If the problem was hair:
- peroxide may loosen surrounding grime
- it usually will not dissolve hair completely
If the problem was soap scum:
- it may help break down organic parts
If the problem was mineral scale (calcium/lime):
- peroxide does very little
If the problem was a solid clog:
- it may bubble around it but not remove it
7. Was it dangerous?
Usually, pouring normal household hydrogen peroxide (commonly 3%) down a shower drain is not dangerous.
However, avoid mixing it with:
- bleach → can create irritating chlorine-containing compounds
- vinegar at the same time → can form peracetic acid, which is irritating and corrosive at higher concentrations
- drain cleaners → chemical reactions may occur
8. What you should do afterward
After using peroxide:
- Wait about 10–30 minutes.
- Flush the drain with plenty of warm water.
- Remove visible hair from the drain cover.
- If drainage is still slow, physically clean the trap or use a drain snake.
For long-term maintenance:
- clean hair weekly
- flush with hot water occasionally
- use a drain cover
- avoid relying only on chemical cleaners
What you may have actually seen after 8 years
The most likely explanation is:
The hydrogen peroxide reached years of organic buildup, released oxygen bubbles, disturbed the biofilm layer, loosened some grime, and temporarily cleaned the upper part of your shower drain.
If you tell me what happened exactly (foam, smell, black water, gurgling, faster drainage, slow drainage, etc.) and how much peroxide you poured and what concentration (3%, 6%, etc.), I can explain the specific reaction.