Every time we use the shower water drips down into the basement. Behind the shower plumbing is the master bedroom toilet. I have taken off the drywall between the two, from the toilet side, and I can't see any leaks in the shower plumbing. In poking around I have since noticed that the flooring around the base of toilet is spongy so the toilet has been leaking for some time. My question is, can the toilet leak because the shower is being used? Actually the leak seems to increase when the shower has just been turned off.
Thanks

"can the toilet leak because the shower is being used?"
No
"the leak seems to increase when the shower has just been turned off."
This leads me =to wonder if the leak isn't caused by faulty and shrinking tile grout where the stream hits the shower wall. This would allow the water to run down inside the wall above the shower pan and collect on the flooring.
This would be my guess instead of a leak in your toilet. Good luck, Tom

It is very hard to diagnos a potial problem with out seeing it, but from my understanding you've taken off the drywall between the two, is the underneth exposed! Can you see the shower drain pee trap to check if that's leaking or maybe the wax ring of you toilet bowl. If that not the case check flow of water do you have a stoppage that sinc it sounds close to (showers 2" drain tied into toilet bend at the base) it coming thur the lowest points tub and tiolet. Leaks are hard especialliy when you don't know what to look for. If you don't secseed don't hesataet to call a professional, it can save you money in water damage and drywall repair! Good luck, James D. Hillman Jr.