When the toilet is flushed, the water drains up into the shower drain? Why?

They share the same drain line which is clogged downstream. If the shower is the only affected fixture the clog is in just this one branch of the drain and not the whole house. Try using a toilet auger but if this can't reach the clog you will have to pull the toilet and root out the drain with a larger, longer snake. Toilet Auger - YouTube

You have an obstruction in the line and needs to be snaked out by a plumber.

Doplumber,

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needs to be snaked out by a plumber.

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This is not the advice people are looking for when t56hey ask a question. If it were we could answer every question with, : Call a plumber". Watch and learn.

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the water drains up into the shower drain? Why?

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You have a blockage downstream from the shower. This can be addressed in one of two ways.
1-pull the toilet and snake the main drain from there, **OR**
2-snake from the lavatory roof vent. Put out on enough snake to reach the base and 15 feet More. Good luck, Tom