They inject the gentamicin directly into the ear drum.
It's an antibiotic that is toxic to the ear.
It destroys the balance organ permanently and there is a small chance of reduced hearing also.
You never fully compensate for the loss of balance on one side just like you can never fully compensate for single sided deafness.
There is something called the vestibular ocular reflex which is permanently lost when moving your head in one direction and it affects both eyes. The VOR is like a gyroscope for your eyes.
When your head moves your VOR adjusts your eyes in a matter of milliseconds.
If you have lost your VOR on one side moving your head one way the VOR works, move your head the other way and your brain has to manually adjust your eyes to point in the right direction. This takes time. It's one of the reasons you will find many ANers do not like driving at night. Bumpy unlit roads being the most challenging as the time it takes to manually adjust the eyes, is slower than the next bump. The constant use of the brain to adjust the eyes can also get tiring. Yes we get used to it..but is that compensation? If severe vestibular problems are ongoing, gentamicin appears to be great option. Just keep in mind that you never fully compensate for a reduced balance system you just get used to it.