Why does my toilet overflow when the shower is running? The water won't shut off & spills over toilet bowl.

The toilet is not clogged up ... it just overflows the bowl because the water won't stop running if the shower is running.

Answers:
Your main drain is plugged between the toilet and the main sewer / septic line. You probably should call a plumber but one thing that worked for me - I got two gallons, yes gallons not quarts of drain opener, poured one after the other in the toilet & let it sit overnight. The next morning I flushed it a couple times to make sure the drain cleaner was out of it. I have never had any more trouble. It is worth a try. It was about $9.00 a gallon at Sam's.

Other answers:
Well i ain't no plumber, but maybe it is time for one...lol.

Never had that happen before, only when a turd gets stuck in the pipe.
Well i ain't no plumber, but maybe it is time for one...lol.

Never had that happen before, only when a turd gets stuck in the pipe.
welll, I ain't no plumma but i's a'say you musta clogged it up wid sum shiiit
You need to have your waste stack snaked. While the volume of water used while flushing the toilet may not be enough to back it up, the volume of water used in a shower would be.
The drain after the shower and toilet is plugged, Snake them both out. Or could be tree roots, call roto rooter.
That bites. maybe theres a clog somewhere further up the system that backs up into the bowl. but you should call a plumber. And I'm curious: what the hell do you do instead of showering now? or do you let the turd water spill onto the floor?

you seriously need to consider a plumber.
The pipe connecting your house to the main sewer line is partially or completely blocked. A professional plumber has the equipment to clean this. It should cost $100-200.
You might want to check/tighten the valve located near the base/rear of the toilet and see if it is open too wide - that can cause the toilet to run. If the pipes are connected, and water backs up somewhere, it will take the path of least resistance, which might be through the toilet drain pipe. This could mean that there is a clog somewhere that needs to be cleared. Try tightening the toilet valve and running the shower first. If that doesn't work, call a plumber.
call a plumber...it doest sound like somthing you can fix your self
Yes there is something clogged in the drain the main reason it overflows to much water pressure and the toilet cant handle it's not a good thing try and buy a snake and go thru the main lines and clean them out if that does not work remove the strainer cover in the shower and stick a non operating snake in pipe and twist buy hand the same with the toilet or they make toilet snakes that don't destroy your toilet
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