The first time John had an unexpected low was terrifying for both of them. Neither of them had seen it coming, and it didn't even have an obvious cause. They weren't in the middle of a chase, John hadn't been exercising, and he was well.
"Sherlock?" John had called.
Sherlock had been in the midst of an experiment studying bacterial cultures when John had called him.
"What?" he called back without looking up from his microscope, scribing notes with one hand while adjusting the view with the other.
"Sherlock," he said again, and there was something in his voice that made Sherlock abandon his cultures and look to his friend.
John was standing just across the island from him. How did he get there without me hearing? He seemed unsteady and pale, and despite being indoors on a relatively cool day, he was sweating.
"John, are you alright?" Of course he's not you idiot, he scolded himself. He shook his head, as if to clear the question from the air.
"I could use some help," he said, looking down at his hand, which was shaking rather more that usual. And was bleeding. Feeling rather slow, Sherlock's brain made the connection between the blood on his finger and the waiting test strip below.
Sherlock carefully directed John's shaking finger to the tip of the strip, which greedily sucked back the blood and began to count down.
"Now sit down," he scolded, leading John from his spot at the counter to his chair. "You're obviously low. You should have started by eating something."
"Yeah," John conceded absentmindedly. He sank into the chair, his legs happy to be free of their burden.
Sherlock rolled his eyes at John and peered at the meter, which had since flashed the warning number, 2.6.
"Low," Sherlock declared, practically throwing the meter at John, who fumbled with it. He yanked the drawer that had been allocated for John's supplies, practically pulling it out of its spot, and retrieved a roll of glucose tabs. Stalking back over to John, he dropped two in his hand.
"Chew," he ordered.
John obeyed, grinning stupidly.
Half an hour later, when he was back in the acceptable range at 4.9, John smiled a weary thank you and declared it one of the most unnerving experiences in his life. Sherlock may have gone shopping after that, picking up glucose tabs in every known flavour, stuffing his pockets with them and hiding them in convenient places around the flat.
Including inside the skull.
John only went a few weeks before he decided he wanted to try a pump.
"It'll fit better with your lifestyle," he told Sherlock. "And therefore, mine."
Sherlock had only sniffed indignantly and trailed along after him when he announced he was leaving for the appointment.
If Sherlock thought the appointment the day after John's diagnosis was long, then this was eons. He was even more put out to find that it was only the first in a series of appointments John would require for 'training'.
"It's not a puppy," Sherlock grumbled on the way out, after hours and hours. "I don't see why they call it training."
John sighed wearily. "Sherlock, you're rather dense. I'm the one receiving the training. And I've told you that you don't have to come."
"I want to," he said petulantly.
"I know," John sighed.
They were more training sessions to go to, and true to his word, Sherlock accompanied John to all of them, even remaining relatively well behaved, although sometimes, he was moody to the point where the nurses would ask if he was feeling low. John always giggled at that as he explained he was the one there for training, rather than Sherlock.
When John did finally get the pump, Sherlock had to admit it was clever. Instead of shots, all John had to do was push some buttons. Much more convenient. Although he had to admit, the tiny tubing was a bit of a draw back, having gotten caught more that once on things in that flat, and that one time in the middle of a chase when it had been forcefully yanked from his skin, leaving a bleeding spot he hadn't noticed until he was back at home, crabby from the unexpected high.
Sherlock was partial to the idea of taping every inch of the tubing to John's skin, leaving no room for snags. John shot that idea down before Sherlock even finished saying it.
He pouted for the rest of the afternoon.