So I really, really don't believe that Sherlock would have been okay with not knowing whether he had chosen the right pill or not. So, here this is! My story on what he would do. Enjoy!
I don't own Sherlock, yada yada...
His hand had been poised, ready. He was going to do it; take the plunge. He couldn't go back now, oh no. He'd come much too far to back out so soon. The pill had been at his lips, and he was going to swallow it—of that he had been sure. He was going to live, and the terrible cabbie genius across from him was going to die slowly and painfully from poison. He could read it in the old man's eyes, his posture—even in the air around him.
Then there was a shot, and the world was momentarily turned upside-down. The man that Sherlock had just outsmarted (easily) was bleeding on the ground, and he would never tell whether Sherlock had made the right decision (even though he was sure he had).
However, amidst the chaos of the crime scene (no Anderson, don't try to organize things; organizing requires thinking and that alone is enough to decimate the more vulnerable brain cells in the room) Sherlock managed to lift the pill off of Sergeant Donovan (her poor knees, why DOES she keep offering to "scrub" Anderson's floors?) and sneak it out under his shock blanket.
It was in the deepest depths of the night that Sherlock tiptoed into the lab of Saint Bartholomew's Hospital to find out if (that) he was right.
It was a tiring process—testing, retesting, disbelieving, re-retesting—and the judge was an unforgiving one. The chemical jury present glared unforgivingly as the detective pleaded his case, giving no feedback but their silent judgment. Even when asked, begged, even, for their decision, they remained stoic in their poison placidity. After tireless hours of research and deduction, however, Sherlock figured out exactly what the verdict would have been.
Death.
The brilliant Sherlock Holmes had made the wrong decision, and, had John not swooped in with his military hand and used it to dispose of Sherlock's adversary, the only consulting detective in existence would have been no more.
Realizing that your pride almost caused your death is quite the sobering experience, and Sherlock's case was no exception. With a grim expression, he lifted his gray-green eyes from the still bubbling flask and stood, staring blankly at the wall. His expression suddenly shifted to one of rage and he hurled the piece of glassware at the wall, watching unsatisfied as it shattered. Throwing things was hardly an effective form of catharsis, and Sherlock knew that. The mind, however, is not always rational.
It was for this exact reason that Sherlock slumped against the door to the research room, effectively jamming it shut. He needed to think, and the quiet whirring of the centrifuge and fume hood drown out the irritating buzz of voices that so seldom occurred at this time of night.
It wasn't until early morning that John realized that Sherlock was missing from their flat. He had been reliably informed by the good Mrs. Hudson that Sherlock was an early riser, and that more often than not John would be awoken promptly at seven by the clanging of dishes in the kitchen. When this was not the case, the doctor became mildly worried. He highly doubted that his new flatmate, however rude and annoying that flatmate may be, would run off before seven in the morning.
Groggily, John turned to his phone to see what time it was. The sudden light from the screen momentarily blinded his still sleepy eyes, and with no little surprise he discovered that he had a text from Sherlock. From two that morning. "Gone to Bart's, be back before you're awake." Closing the message, John rolled his eyes. Right. Before he was awake.
The man half stumbled, half walked into the kitchen. No Sherlock. In the living room, the outcome was the same. He glanced towards the clock. It was nearly ten. John sighed and scratched his head before stretching. Looks like he'd have to go after Sherlock to see if he was alright—can't have a consulting detective on the loose in London streets: it's simply not safe.
When John finally arrived at the laboratory, it was half past ten. He glanced in the windows and frowned. No Sherlock. When he pushed open the door, however, he found it no more mobile than within a few inches. He leaned against it, and whatever was blocking the door groaned briefly (definitely Sherlock) then stood up, sending John tumbling forward onto his knees.
"What the bloody hell was that for?" John snapped as he pushed himself up to full height, brushing off his palms and knees. He glanced at Sherlock through his discontent, and was surprised to see the taller man looking haggard and worried. "Are you alright, Sherlock?"
"Hm?" Sherlock's eyes snapped to alertness and the concern that had been blatant on his face disappeared. "Oh, yes. Of course."
The doctor glanced over Sherlock quickly and saw no signs of immediate trouble, and thus decided to take Sherlock at his word. "Right," he said, "So why did you decide to come here in the middle of the night? You doing another one of your 'experiments?'"
At that, Sherlock's eyes simultaneously lit up and assumed a slight shine of worry. "I can normally do my experiments in the kitchen," he began, "but I ran into some trouble when I encountered the specific alkalidity of this particular substance."
"Alkalidity?" John asked, "Is that even a word?"
Sherlock paid no heed. "This substance is an alkaloid, which is a very broad field and yet very narrow. It consolidated my selection of possible substances to one out of—" He paused his lecture to scratch his head thoughtfully. "—around two-hundred possibilities, which is a lot smaller than it seems. The powder could have been any one of those two hundred things—a hallucinogen, anti-carcinogen, aphrodisiac, antiarrhythmic, poison—so I simply had to continue my analysis where there was the proper equipment for the extension of my understanding—"
"Hold on," the doctor interrupted, and Sherlock looked up at him with mild surprise, "what's all this about?"
The consulting detective gave a disgusted look—the one that implies that the listener had been doing not enough thinking and a little too much drooling—before answering John's query. "The pill, John, the pill! The one that you stopped me from ingesting not…" He glanced at his watch. "Fourteen hours ago—fourteen? Has it really been that long?"
John sighed. "Yes, Sherlock, it has." He ran an exasperated hand down his face. "So what does this have to do with the pill?"
"John," Sherlock began, staring seriously into John's blue eyes with his own piercing green, "I almost ate a pill that could have killed me. You can't expect me to accept not knowing whether I was right or not."
Suddenly it all clicked, and John grinned. "You had to satisfy your curiosity." His grin widened and he pointed at Sherlock meaningfully. "You thought you might be wrong."
The detective waved a hand to dismiss the idea. "The chances of that were one in two; there was always a possibility I could be wrong."
"No." John cocked his head. "You wouldn't have risked your life on a fifty-fifty chance. You thought you were right. You knew you were right. Why did you have to test it?"
The brunette gave John the "please don't ask" look that he assumed on occasion. John held his gaze.
"I had to be sure." Sherlock turned away and ruffled his hands through his hair before turning back and practically pleaded with John to understand. "Don't you understand? The cabbie died before I had the chance to find out whether I was right or wrong, so I had to go about finding out other ways."
"So?" John stared intently at the manic hobbyist, waiting for an answer.
He stared back.
"Well." The detective ran a hand through his wild black hair. "I did a few tests—just a few simple ones. I determined that the substance was an alkaloid first, which narrowed it down from infinity to around two-hundred. I then determined—"
"Yeah, yeah," John interrupted him, "That's fantastic, Sherlock. Did you pick the right pill, though?"
The taller male stood in silence, staring at his shoes (shiny, old, the dirt shows where he's been. Note to self: clean shoes). "I…" He faltered. "It was aconitine."
The doctor gazed blankly at Sherlock. "It was what?"
"Aconitine!" Holmes threw up his hands in frustration. "It's aptly called 'the queen of poisons.' Deadly almost instantaneously, paralyzes the lungs. And I almost ate it." He slammed his palms down on the lab bench and glared forward at nothing.
John stood in stunned silence. The consulting detective could be wrong—that much was obvious with the blunder about Harry—but wrong enough to die from it? And defeated, nonetheless. That had to hurt.
Watson didn't know how to react to this. On one hand, the idiot had nearly killed himself for the thrill of a game, and on the other hand, the idiot had nearly killed himself for the thrill of a game.
John put an awkward yet consoling hand on Sherlock's shoulder. "Sherlock." The doctor cleared his throat. "Risking your life over a case is stupid."
The man in question looked up at John through his eyelashes, as if to say "no shit."
John gave him a chastising look in return. "But you acted upon what you thought was right."
Sherlock stared at the blond for a moment, whether in shock or in gratitude was unclear. Then, with lethargic movements, he lifted himself off of the bench and towered over John, expression unreadable.
John fidgeted, and glanced around the room quickly before gluing his eyes to Sherlock's in a successful attempt to hold his ground. "You were stupid."
The corner of Sherlock's mouth twitched.
"But it doesn't matter because I shot him."
Sherlock continued staring at John intently, before allowing himself a small smile. "Thank you, John."
The doctor did his best not to physically recoil in surprise at the admission of thanks. "You're welcome?"
Holmes gave a heartfelt chuckle which Watson returned wholeheartedly. "Let's go home," John said, "your skull seemed to be missing you when I left."
"Indeed," the detective returned, "indeed."