Sherlock, true to his word, had not touched any of John's supplies yet. But two months in, John on the pump and his blood sugar levels fairly steady, he figured it wouldn't hurt to do some experimenting.
It had started off harmless, just testing his blood sugar with the bonus meter John had gotten, muttering something about chases and Sherlock dragging him off without giving him a chance to grab everything. So now John had a tiny meter that lived in his coat, and the normal one he used most of the time. That was all well and good, but his interest in that didn't last long. It wasn't as if Sherlock's blood sugar was changing all that much, his pancreas doing what John's couldn't, however dull the task may be.
Until he had the realization that it could change, under the right circumstances.
There was an entire section of his brain screaming at him that this was a bad idea, but he ignored it in favour of the fascinated section, which urged him to do it.
He'd calculated the dosage carefully, one that would dip him into the low range, but only barely.
Sherlock was perched on the couch, the meter sitting on the table in front of him, along with an army of glucose tabs, and the orange glucagon kit, just in case.
He checked the text from John, sent seven minutes ago, stating he would be home in ten.
It was now or never.
Sherlock took a deep breath and plunged the needle into his abdomen where he'd wiped it with an alcohol wipe. It wasn't the needle that scared him, he'd had enough of those to last a lifetime, but it was the effect that the contents of the needle would have on him that he was worried about.
After all, John, the man who'd been shot in a war, then come home to London, only to be stabbed, strangled, and shot again, had claimed a low was the most unnerving experience of his life.
He recapped the syringe, with fingers that were already beginning to tremble, whether that was an actual symptom or simply psychosomatic at this stage was hard to tell.
He focused on putting a test strip into the meter, pulling the device closer. As he pricked his finger, he noticed a definite tremble, a real one. It made it hard to hold his finger steady and direct the droplet of blood towards the tiny target. He made a note to look into that.
While he waited for it to count down, he absentmindedly wiped the leftover blood from his finger on the couch, only belatedly thinking that was probably a bad idea, but his mind was foggy, and obviously not thinking clearly at the moment.
The meter flashed a 3.2, and Sherlock looked at it for a moment, marvelling at the magic. But then the rational part of his brain (the one that told him not to do this experiment) yelled at him that he had to do something about it, and Sherlock had to admit it was probably right.
He reached for the glucose tabs, cursing their packaging, and made a note to look into those as well. He shoved one in his mouth, crunching hard, the taste of chalk filling his mouth. He made another note to make them taste better, somehow. He finished one, and shoved another one in his mouth, the distant sounds of a cab door slamming and the front door opening. John was home.
Sherlock's brain, the entirety of it, panicked, clumsily shoving evidence under couch cushions. How had he not prepared for this eventuality? Stupid, stupid, stupid, he cursed himself.
He lay back on the couch in his thinking position just as John came in the door, surveying the room suspiciously.
"What are you doing Sherlock?"
"Thinking," he replied, working hard to not slur the words around the chalky flavour in his mouth.
John shook his head and went into the kitchen, flipping the kettle on.
"Tea?"
Sherlock hummed in response.
John sighed and reached into the fridge for the milk, finding instead the insulin bottle that Sherlock had taken his sample from. It had moved. Slightly.
Damn.
John returned to the living room, holding the bottle in his hand."Sherlock, have you been experimenting with the insulin?"
"Of course not," he scoffed, hoping it sounded convincing.
John scrutinized him. "Liar. You're paler than usual and despite trying to hide it, you're trembling." He raised an eyebrow. "Unless, of course, it's psychosomatic."
"Of course it's not," he snapped.
John threw two more glucose tabs at Sherlock. "Chew," he ordered, digging the meter out from the cushion, instinctively knowing it was hidden there. "Hand," he demanded, brandishing the lancing device like a weapon.
Sherlock frowned, but flung an arm at him, not even blinking as John attacking his finger, possibly with more force than necessary, squeezing a droplet of blood onto the strip. It was easier to do with steady hands, and John's were. (Under pressure right now...)
"What was it before?"
"3.2," Sherlock replied, not even bothering to ask how John knew he'd tested recently.
John nodded. "Headed in the right direction." He held the meter up for Sherlock to see, 3.6. "I'll get that tea."
Sherlock nodded, just laying there on the couch, feeling rather like crap, and understanding why John wished to avoid these. They sucked. And to top it all off, he could feel a headache coming on, one that he suspected he'd been unable to get rid of that day. Just punishment, perhaps.
John returned with the tea, and Sherlock could tell it was sweeter than usual. "Don't," John warned.
Sherlock didn't.
After the tea, John grabbed a different finger an attacked it, seeming content that the number was up to 4.5. That was when he sat down on the table to scold Sherlock.
"You are a daft bastard," John had told him, his voice steady. That was perhaps the most terrifying part, that he wasn't yelling, or even raising his voice, but was perfectly calm.
"I just wanted to know what it was like..." he whispered.
"Why could you want to experience that?" John sounded completely outraged.
Sherlock was silent. He could see John thinking that over, and it was like a light bulb went on. John's thinking really was that obvious.
"Oh..." he said faintly. "Sherlock..." he trailed off. "I appreciate the thought, but it's really not necessary. And that was stupid, doing it like that, alone."
"You're often alone," he pointed out.
John sighed. "Yes, but I don't have a choice. You do, and you made a pretty stupid one."
Sherlock pouted.
"But I know how your big stupid brain works, so I get why you did it. I'm just saying, don't ever do it again. I have enough to worry about without stressing over whether you're shooting up insulin at home in the living room to see what it feels like."
"Fine," Sherlock conceded.
He knew what it was like now, and as far as he was concerned, would be fine if he never experienced it again for the rest of his life.
Life went on. It was the same as life before, except there was more eating, the occasional blood droplet on furniture, and chalky glucose tabs thrust at you from every angle when you finally managed to say "I feel low". Of course it wasn't easy, but nothing ever was.
Especially when Sherlock was around. But to everyone's shock, he was more understanding than impatient, more protective than dismissive, and was the first to suggest that perhaps John was feeling low when he couldn't see that the dead woman was strangled, not shot.
And in John's books, that was a caring Sherlock.