Plumbing and drains

Why does water come up in the bathtub when I flush the toilet?

The toilet and tub probably share a drain downstream, where a restriction is redirecting wastewater toward the lower opening.

Urgency: urgent

What this means

Limit water use and determine whether other fixtures are also slow.

Do not run laundry or a dishwasher
note every affected drain
one careful flange-plunger attempt is reasonable only if no other fixture is affected.

Stop using water and call a drain professional if several fixtures react, sewage appears, or a lower drain overflows.

Contact the matched licensed or qualified trade if safe checks do not resolve the issue.

This answer combines HomeSOS's structured triage with safety boundaries supported by a public source. It does not replace inspection.