After pouring 1.5 bottles of horrid caustic chemicals down the drain to no avail, I found I was able to clear a tough clogged shower drain with just the empty bottle:
- pour kettle full of boiling water into drain
- stuff overflow drain with plastic wrap (so that pressure is directed toward the clog, not out the top)
- invert empty draino bottle placing mouth around drain, forming a tight seal
- squeeze bottle rapidly and forcefully
Don’t get boiling water in your eyes.