Older adult participants with treatment-resistant depression will be randomly assigned to a Step 1 medication strategy.
* Adding aripiprazole to current antidepressant medication
* Adding bupropion to current antidepressant medication
* Replacing current antidepressant medication with bupropion
If depression is not relieved at the end of 10 weeks, or if participants do not qualify for Step 1, participants will be randomly assigned to a Step 2 medication strategy:
* Adding lithium to current antidepressant medication
* Replacing current antidepressant medication with nortriptyline
All medication strategies will be offered in collaboration with participants' own physicians with the the research team providing support and guidance.
After treatment in Step 1 and/or Step 2, participants will enter the Continuation Phase to assess long term follow-up outcomes for 12 months.
Participants in this clinical trial will also be asked to participate in an additional study to gather imaging and biomarker data.
