A/N - Alas, I don't own the characters or anything else - and make no profits from this story. Now that's out of the way... enjoy!

John sat on the kitchen table swinging his legs while he faced his pipette-wielding flatmate and frowned. "Tell me again why I'm doing this Sherlock?"

"It's for a case John, I told you," Sherlock replied, managing to sound both petulant and outraged at the hint of fed-up-ness in John's voice. "Surely even you can recall? I told you about it yesterday and you promised you'd help. It was the incident last Thursday from the website, the one with the woman and the man who definitely wasn't her husband…and the locked bedroom."

"Oh, I remember the one," John brightened, relaxing his defensive posture slightly as he recalled a distinct lack of gore at the scene which meant this experiment was less likely to end in physical harm for him too, "Where both she and the husband swore blind she'd never met the man she was found in bed with, despite her declaring her undying love for him when the hubbie caught them. Didn't she think she'd been drugged?"

"Exactly!" Exclaimed Sherlock, "and she was telling the truth - I picked up no indication of deliberate deception. Plus of course there was definitely something in her blood. So, given all the evidence, when you have eliminated the impossible, whatever remains - "

"- However improbable, must be the truth," John interjected, smirking at Sherlock's catchphrase.

"Yes, John," Sherlock looked down at him frowning at the interruption, "Improbable but not impossible that the couple… uh," Sherlock consulted his notes, "Mr and Mrs Warwick, were correct and there was some kind of chemical prompt for her behaviour rather than the more prosaic outcome that she was simply lying to hide her infidelity."

"So, and I know how you hate it when I repeat myself… why am I doing this?"

"Jooohhhnnn" Sherlock whined, now definitely in full pout in an attempt to manipulate John into capitulating, "You told me you'd help. No drug on the market matches the chemical composition of the one in Mrs Warwick's bloodstream, nor can I find anything readily available which would produce her exact symptoms. You agreed you'd help me test replications of it and prove it made her act that way. A marriage is at stake here John!" He finished with a dramatic flourish, waving the pipette as though conducting an orchestra.

John gave a very inelegant snort at that declaration, knowing full well Sherlock cared far less for the sake of the Warwick's marriage than he did for the lure of solving the puzzle of an unidentified chemical solution which could provoke such irrational behaviour.

"It really is very simple," Sherlock continued, "We know the chemical makeup of the drug but not the quantities or the preparation method. If I can just monitor the outcomes in a test subject I can ascertain the exact composition of the drug and validate Mrs Warwick's claims. Or not."

"So… let me get this right… you are going to dose me with variants of the chemical solution found in her blood until you find one which will make me believe myself completely in love with someone, albeit not forever, and then use that to prove the wife wasn't intentionally cheating on her husband?"

"Really John, there is absolutely no need to repeat my words back to me - I know exactly what I said." Sherlock continued reprovingly. "And anyway, the concentration I'll be using will be substantially diluted. To be honest I doubt you will even notice the effects."

John sighed loudly, but his heart wasn't really in it. He had agreed to do this and despite all evidence to the contrary (Baskerville came instantly to mind) he trusted Sherlock not to deliberately harm him.

"If I'm doing this I want some ground rules in place," John told Sherlock sternly, batting away the detective who was already approaching with the pipette outstretched in recognition that John had given in. "You are not drugging me until you agree."

"Fine. Whatever you want." Sherlock agreed with him dismissively, knowing full well he would follow John's rules only as far as they coincided with his own plans. Not that John needed to know that.

"I'm serious Sherlock! If you don't listen I'll walk away now and you can go back to wild speculations and a distinct lack of scientific proof for your theories."

Sherlock huffed and sighed and did his best flouncing away from the table, but eventually agreed to listen to John's conditions.

"Ok. First off, how long will each dose last?"

Sherlock consulted his notes and muttered under his breath, "Given body weight and dilution of the compound, minus food consumed in last two hours, add in half-life factor of five…" He looked up at John, "12 hours - 24 at the absolute most."

"That's manageable. Then rule one is that you don't leave me alone during the time I'm under this compound's influence. No running off to investigate some fascinating double murder while I'm unable to control my own mind."

"That seems… reasonable I guess." Sherlock conceded, although he was a bit put out at the thought of potentially giving up a good murder.

"Second. Hmmm. No deliberately involving other people. I don't want our friends discovering me in compromising situations, or discovering I actually consented to you drugging me."

"Yes John," muttered Sherlock sulkily. Really, John was taking all the fun out of this now.

"Thirdly - no drugging me without my consent. We agree when and where together. No slipping it into my coffee when I'm not looking."

Sherlock had the grace to look slightly abashed at that one. John had been furious after the whole HOUND incident.

"Fourthly… You share all the results with me afterwards. No secrets." John paused, trying to recall the details of the Warwick's case. "Did she remember everything when she was drugged?"

Sherlock nodded slowly, "She did… but she remembered more concept than detail - sentiment and emotion over fact and observable evidence. Mrs Warwick told me she recalled the pleasure and sensation of being completely head-over-heels in love with the stranger, but was unable to tell me what she had eaten for dinner."

"Then I definitely want to see all your notes after the effects of each dose wears off."

"Yes, fine. Are we done now?" Sherlock lost his patience with all the rules and boundaries. He was itching to get started and see how close he had got with his first composition.

"I suppose so." John had dropped back to his mock-stern 'I'm doing you a huge favour here so appreciate it' voice, and Sherlock knew that meant he was only being so reticent because he didn't want to appear keen. But John was a doctor, a scientist too, and he secretly loved a good experiment almost as much as Sherlock. Even John had to admit he was a little curious as to how much effect Sherlock''s concoction would have on him.

"Good. Well, it is midday now… shall we start? I'm expecting the effect to become more pronounced after an hour or two, peaking in hour 5, then fade out over several hours. The first dose is relatively low in strength so should be completely out of your system by the time you wake up in the morning, if not before."

John jumped off the kitchen table and stood by Sherlock, looking down at the laptop screen as Sherlock entered data into his spreadsheet documenting the effects of the drug on John.

"We'll start with two drops administered orally and see how we go. We can always adjust the delivery method and the quantity for next time if this isn't conclusive enough. Now, open your mouth and stick out your tongue"

John rolled his eyes at the command but obligingly did as Sherlock asked. Sherlock took the pipette and carefully administered two drops onto John's tongue then wandered off into the lounge with the laptop to update his notes. John was left standing in the kitchen on his own, wondering what he was supposed to do now. Only one way to find out…

"Now what?" John asked, hovering in the kitchen door.

Sherlock glanced up from the screen to meet his eyes. "Now John, we wait."

A/N - Reviews, comments and all other feedback most welcome. Next chapter will be up tomorrow x