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John and Sherlock had been fighting quite a lot recently.
Well, that is to say, John had been trying to fight whilst Sherlock mainly accepted whatever John might have put to him without too much fuss.
John had simply been irritable for the past while, and he didn't know why.
He especially didn't understand why he was taking it all out on Sherlock, the man whom he loved with all of his heart.
And, yet, here he was, shouting once again at Sherlock over something completely stupid.
John let out a frustrated laugh, feeling incredibly frazzled.
It just didn't make any sense to him; there was no trouble at work, the relationship that he had with Sherlock (with the exception of the recent outbursts) was wonderfully easy, and life, in general, was grand.
Sherlock watched him languidly, knowing full well that it wasn't his fault that John was irate.
He also knew that there wasn't much to do that would quell his temper, and so he would let John wind down on his own as one might do with a child.
After another ten minutes, John sat down on the sofa, putting his head in his hands and looking downright miserable.
Sherlock sat across from him in his large black chair, the silence a little striking after all of that noise.
Fortunately, the flats were rather soundproofed; whether it was because John was shouting or another activity might have been taking place, that facet of the property had proven rather useful.
After John had fully calmed down, Sherlock moved over beside John, wrapping his long arms around the shorter man in a consolatory fashion.
He kissed the top of John's head, not understanding what was wrong, but knowing how to soothe the aftershocks of upset.
John felt embarrassed and out of control.
No matter what he'd tried, nothing had worked to cool his temper.
Sherlock sniffed, noting something different in the way that John smelt.
He sniffed once more, and John pulled away.
"What are you doing?" He asked, wondering if he ought to be offended.
He had showered that morning, so it wasn't as though he should stink.
Sherlock had a look of understanding on his face.
"Your multivitamins, John," Sherlock stated in realisation.
John shrugged. "What about it?" He asked a little touchily.
"It's so simple!" Sherlock replied, his nose wrinkling characteristically. "Both of us should have realised it much sooner. "You've recently started taking a daily multivitamin; very few people would react so strongly, however, I suspect that the vitamin D has been raising your testosterone levels, which in turn, has been raising your temper and libido."
John blinked.
What Sherlock said made sense, though John hadn't thought for a second that it could have been his multivitamin that had been causing the issue. After all, it was incredibly rare.
"We can do a blood test right now to determine whether or not I'm correct if you like." Sherlock offered, wanting to have it confirmed.
He always did like proof that he was right.
John wanted to be annoyed at this, but couldn't help but be ever so slightly amused.
"Fine." He relented easily, and Sherlock got his kit and set about taking a blood sample.
Naturally, Sherlock's hypothesis had been proven correct, and John stopped taking the vitamins.
Within the week he was back to normal, and things were easier again.