John was speechless. When he found his voice, he meant to shout, but he choked on the word as it came out. "What?" he managed. You meant to take the poisoned one?
Sherlock turned and walked to his favorite chair. He sat in it, steepling his fingertips. He pressed them to his lips thoughtfully.
"Remember what was happening that evening, John," he said. "Lestrade and his people had invaded the flat."
"Um, yes, you were quite put out with him, as I remember," John said, struggling to focus on the continuing conversation and for a normal tone of voice. My God, Sherlock.
"He was pressuring me, accusing me of withholding evidence," Sherlock said, frowning at the memory. "And having all of his people handle my things, ruin my experiments, rummage through my papers."
"You were withholding evidence," John responded, remembering. "You ought to treat that man with more respect, Sherlock. Your – our - livelihood depends on him agreeing to let you in on cases, to have access to crime scenes. You need him."
Sherlock smiled. "The man needs me, God help him. He was just messing with me, trying to put me in my place with a petty power play. Unfortunately, however," he said, serious again, "although he didn't know it, he really was about to find evidence of illegal drug use."
"Yes, you told me later it was a marijuana sample that you were analyzing for a case. I chose to believe you. It wasn't marijuana, though, was it, and it wasn't just a crime scene sample," John said, searching Sherlock's face. Sherlock didn't answer. "What were you using," John asked levelly. "What are you using?"
"It doesn't have a name, only a long chemical formula," Sherlock replied.
"It's known by a chemical formula?" John asked, surprised.
"No, John, it's not 'known' at all, except by me, and I don't have a name for it," Sherlock snapped. At John's look, he said, impatiently, "Yes, John, it's a drug of my own invention, of my own manufacture."