What does "as needed" mean?

[SzAdmin]

Past discussions on this topic:

What's the PRN?

A fifty year old man in my circle takes conscientious care of his depression-centered mood swing stuff. Specifically, he’s into regular exercise and hopes to make sleeping and eating as healthy as possible given his inclinations and abilities. He also has a relationship with a doctor who treats him with medications for depression, mainly. Now, for three days running, he is in depression; he asks me, “What’s a PRN for my current situation?” Folks help me out with an answer to this fellow! Jayster