“I have a client who is taking a high dose of a stimulant, as well as a painkiller and sleeping pills. I think that she’s addicted to the stimulant. Is inpatient detox the only course of action?”
If your client is on a stimulant medication, plus a painkiller and sleeping pills, there is more going on than ADHD. Due to the complexity of her treatment, I could not conclude from your question that she is indeed addicted to the stimulant. It might be that the sleeping and pain medications are sedating her and that the stimulant helps her stay awake.
If the prescribing physician has a good relationship with her, he can help her wean herself from each medication to establish a new baseline. This might be tried as an outpatient procedure. If not, she might require a brief hospitalization to get her off all the medications and to establish a new baseline from which to treat her problems.