I woke up on Tuesday morning, and all seemed dark. This was out of keeping -- therefore I asked myself a diagnostic question I often use in counselling: “What changed?” On Monday, I took a single big punch of penicillin, against rheumatic fever. I checked the penicillin. According to statistics, if one takes a short course of penicillin, especially if one is 40+, one is at risk of briefly dipping into depression. The following morning I woke up fine. OBSERVATION: The link between penicillin and depression is not merely based on symptoms. The mechanism is now well understood. I am a bit taken aback by this.