Everything's a toxin at a high enough dosage. And even at that dosage they can still be therapeutic. See: chemotherapy, as an extreme example.
With botox specifically, it has applications outside of just making lizard-faces. Off the top of my head, it's used as a treatment for migraines by injecting it into the back of the scalp - any swelling/filling effect it has is concealed by the hair anyway, unless the patient is bald.
