The setting: the character is a cop whose personal life recently nosedived and whose professional life is following close behind, so there is some moderate depression already. He works in a busy police station, so someone can probably slip in easily.
The drug/s needed: something that can be slipped into food/drink without being noticed too much, and that could seriously deepen his depression to the point of suicidal thoughts. It could be several drugs, or just one. Something that causes memory lapses is a plus.
Well, hope this was all according to the rules - and any help with this is very greatly appreciated!