Answer: Iproniazid was originally developed as a treatment for tuberculosis, but was soon found to have antidepressive function.
https://www.medscape.com/viewarticle/848709
Iproniazid was developed to resemble a related compound, isoniazid. Isoniazid, first developed in 1952, is a frequently prescribed drug for the treatment of tuberculosis. It is cheap to manufacture and is easily distributed, making it a powerful tool in the developing world to decrease the spread of TB. Isoniazid acts to fight tuberculosis infection by inhibiting production of the cell walls of Mycobacterium tuberculosis, the disease-causing bacteria that causes tuberculosis.
Iproniazid however was found to be ineffective. There are profound side effects to the drug, such as liver inflammation, nausea, vomiting, and numbness. Headache, poor concentration, and a decline in memory have been observed with isoniazid (Isoniazid-induced psychosis.
Alao AO, Yolles JC). It interacts dangerously with alcohol. It also decreases red blood cell and white blood cell count.
Interestingly, iproniazid was shown to have a therapeutically useful side effect for treating depression. Dr. Joanna Moncrieff describes the use of iproniazid and the related drug isoniazid for decreasing measures of depression in clinical trials (The creation of the concept of an antidepressant: an historical analysis.)
The reports of antidepressant function of iproniazid has led the way for researchers to consider drugs with a similar pharmacological action as a treatment for depression.