The Reason Is You (слэш)

John/Sherlock. A/O Verse. Alpha!John, Omega!Sherlock, Sherlock BBC Prompting Meme
"John really likes Sherlock, but he's not sure he should move in with an Omega (Sherlock is one). He's worried that things will be stressful for both of them.
Sherlock, however, tells him that it won't be a problem since his body doesn't work properly and he's never had a heat at all, not a single one. He has a medical condition and his hormones prevent him from being a proper Omega, and therapy didn't work either (not that Sherlock minds. His nature doesn't distract him from really important things, he doesn't want a relationship or family anyway, so he's perfectly fine with his condition).
So they start living together. But John's hormones start affecting Sherlock eventually. Not right away, but, gradually, something changes in Sherlock's body. And one day he goes into heat, the very first one. He panics. John tries his best to soothe him and control his own instincts that tell him to take the virgin Omega immediately."