Amy keeps biting her nails (when she's not picking at her lips, sometimes until they bleed).
Stella bought some bitter-tasting liquid (with denatonium as the active ingredient, I believe) to apply to the nails, and Amy seems to have accepted it pretty well.
Stella said that she explained to Amy that she wasn't intending to punish Amy, but that she wanted the nails to grow nice and healthy and that this would help them.
Amy occasionally even asks for "her nail polish" herself when she catches herself biting her nails. It's mostly unconscious anyway, so the bitter taste helps her realise how often she's doing it.
I'm glad she's taking this so well.