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Apparently nitroglycerin tastes like toasted caramel. Katsuki found that out when he asked Izuku why he was always so persistent with his mouth. Usually, consuming any amount of nitroglycerin is dangerous, but Katsuki's been inadvertently micro-dosing Izuku since childhood, and there hasn't been any negative side effects yet.
It seems there won't be any negative side effects, considering how often Izuku's mouthing at Katsuki's neck, his thighs, his hands, pretty much anywhere he sweats, licking up the taste like it's golden honey.
Just like he is now, his arms wrapped around Katsuki's torso, face buried in his neck to lick and suck up the sweet sweat of a hot summer night. He'd begged for ten whole minutes, ("please let me taste you" and "I'll be good, I promise") before Katsuki had let him. (He would've let him anyway, he just liked hearing Izuku beg.)
Katsuki likes it too, honestly. He loves how feral Izuku gets over him once he gets a taste. On multiple occasions, blackwhip has been used when he hasn't stayed still enough for Izuku's liking. He's not bothering fighting back this time though, he's too hot between the summer heat and the body trapping him against his mattress.
Izuku's sucking at his throat, right in the hollow that makes his legs shake. Katsuki's hands are buried in his hair, holding him close, but loose enough to let him move where he wants to.
He chooses to kiss down Katsuki's chest, licking the pooling sweat under the curve of his pecs, then licks back up. His tongue flicks over a nipple, earning a groan from Katsuki, then he sucks the bud into his mouth, a high, needy sound escaping Katsuki before he can bite it back.
Izuku just moans back, biting at Katsuki's pec before kissing higher again. Katsuki moves his arm up, aiming to run it down Izuku's back, but before he can- "Ew, Izuku, did you just lick my armpit?! What the fuck?!"
Katsuki jerks back, giving Izuku a grossed-out look, but he just smiles, unashamed, and nuzzles back in with a deep inhale. "Mmn, yeah, I did, you taste good. Like candy. Like you." He licks again, and Katsuki yelps, shoving his head back with both hands.
"Fucking gross idiot." He huffs out, but it's laced with affection. Izuku's smile grows at that, and he leans up and in, but Katsuki stops him with a hand over his mouth. "You are not kissing me after you've been licking up my sweat for the past ten minutes."
He feels Izuku pout against the palm of his hand, but he pulls back anyway, burying his face in Katsuki's neck again. Katsuki's hands gently tangle in Izuku's hair again, not guiding him, just holding him close.
He thinks that's what Izuku likes most about this, if his usual rambling is anything to go off of. Something about "can't believe I get to be so close to you" and "I love getting to be the only one who gets this" and "Kacchan's so good, so sweet" that Katsuki barely pays attention to anymore with how many times over he's heard it all.
Katsuki's brought out of his thoughts by a particularly rough bite on his shoulder, and he lets out a sharp hiss, tugging Izuku back by his hair. "Fuckin' ow, ease up, Izu." Izuku huffs in annoyance, but when he leans back down, he licks over the bite mark, soothing the grooves he left behind with his tongue.
Izuku kisses a few more times, slowly moving down over Katsuki's shoulder and bicep before biting harshly into the hard muscle there. Katsuki groans, tugging Izuku back again, and trying to give him a stern look. "What the fuck did I literally just tell you about taking it easy?"
Izuku whines, trying to tug himself out of Katsuki's grip, trying to get close enough to taste again, but Katsuki doesn't let him go. "Fuck, I can't help it, you just taste so good, Kacchan. I'd eat you for real if it wouldn't kill you."
Huh. That's new. Not the comment about how good he tastes, Izuku's been doing this whole routine for, like, a year now, and he's even gotten tastes of Katsuki's blood when he's bitten too hard. But the reference to cannibalism was new. Huh.
Katsuki's grip on Izuku's hair must've loosened during his stupor, because he's kissing down Katsuki's body again, licking into the dips of his abs and biting at his hip bones. His hands are gripping Katsuki's thighs with a force that is definitely going to bruise, but Katsuki's not complaining. So he's a bit of a masochist. Sue him.
That little comment seems to have opened a floodgate though, because now Izuku's rambling, but not the usual, mostly wholesome if not incredibly horny comments he makes.
"I'd like that, I really would, I'd love to bite you so hard you bleed, wanna lick it up. I wanna tear into you, wanna make you such a mess. Kacchan's sweat and blood already taste so good, I bet his flesh would be even better."
Maybe Katsuki should put a stop to this before it gets out of hand. Because push comes to shove, he knows that if Izuku really asked for it, he'd let him rip him into pieces. It doesn't seem like Izuku's actually asking though, just rambling, probably a little high of the nitroglycerin he really shouldn't be consuming this much of.
A particularly sharp bite to his thigh has Katsuki jolting out of his thoughts, Izuku's teeth digging in deep, and he's grinding them like he's actually trying to get a piece. Usually this wouldn't concern Katsuki, Izuku does that when he wants Katsuki to bleed, but after the comments he was making, it admittedly makes him a little nervous.
He can't bring himself to make Izuku stop though, not when he's staring dazedly up at Katsuki, eyes half closed and glazed over, like there's nowhere else in the world he'd rather be. Katsuki knows he wouldn't rather be anywhere else.
A sharp gasp tears from Katsuki's chest when Izuku finally breaks skin, a deep moan rumbling from his chest as he starts sucking on the bite, drawing out more blood to swallow.
With a breathless groan, Katsuki flops back on the bed, eyes shut tight as he lets Izuku work. He's sort of glad Izuku didn't keep biting, that shit hurt. He also sort of wants more. He's almost painfully hard, has been since Izuku started this whole thing, and watching Izuku suck his blood definitely pushed him closer.
As if he'd been reading Katsuki's mind, Izuku bites back down, right into where he did last time. Katsuki barely manages to avoid choking on a borderline scream, grabbing hard at Izuku's hair and trying to pull him away.
Izuku doesn't budge. Instead, he lets dark curls of blackwhip unfold from his fingertips where they grip Katsuki's thighs to keep them open, wrapping around his lower half to keep him still. His teeth dig in harder, past the soft, vulnerable flesh and deeper, straight into the muscles jumping painfully under the sharp pain.
Katsuki sobs, writhing the most he can and trying to buck Izuku off to no avail. He writhes under the tendrils holding him down the best he can, which isn't much, any words caught in his throat as Izuku tears into him.
It hurts, it hurts so fucking bad, but it feels so good, he's so hard, any fucking friction at all and he thinks he'd actually cum on the spot. Izuku eases up for a second, and Katsuki takes the reprieve to pant heavily, trying to catch his breath. Then, the soft feeling of blackwhip sliding underneath the hem of his boxers, gently curling around his cock, squeezing and pressing in all the ways he likes.
Katsuki whines loudly, lifting his head dazedly to look at Izuku. The sight alone almost undoes him. Izuku still licking at Katsuki's thigh, blood and spit smeared across his face, dripping down his chin, Katsuki's thigh, onto the bedsheet. Izuku's rolling his hips into the bed, and he looks like he's on cloud nine, eyes fully unfocused and glazed over.
Izuku presses his tongue just a little harder against the bite mark, and Katsuki whimpers, tossing his head back onto the bed with another small sob. Fuck, he must look a mess, tear stained and bloody and sobbing, but he's so fucking close, cock throbbing against the delicious pressure of blackwhip around him.
Izuku growls under his breath, and then he bites again, deep and rough, and blackwhip swirls and presses, and Katsuki's head is spinning, and then he's gone.
Katsuki comes to with no restraints on his body, just a wet washcloth pressed against his thigh. He doesn't open his eyes for a long moment, just focusing on the deep, throbbing pain radiating from the bite, not wanting to attempt sitting up yet in case he's lightheaded still.
He does open his eyes when Izuku starts gently working his boxers off, groaning as the light hits his eyes. Izuku chuckles, pressing an apologetic kiss below his belly button.
"Hey, welcome back. Lost you there for a second. You feeling okay?" Izuku asks softly, taking another damp washcloth to the sticky mess between Katsuki's legs. Katsuki just groans again, but nods slowly. He doesn't feel dizzy or anything, and now that he's really focusing, nothing in his leg feels too wrong, just painful.
Izuku presses a soft kiss over the bite before shuffling back to get himself cleaned up. He had already wiped the blood on his face and neck away while Katsuki was unconscious, but had apparently left the mess in his shorts for after Katsuki was taken care of.
Katsuki's really not surprised by that, Izuku tends to get off untouched when he gets lost in Katsuki like that, but neither of them complain. Less either of them have to do when they get like this.
After flitting off to the en suite to get the first aid kit, Izuku comes back to examine the bite wound (because yes, his flesh is torn and open, it's a wound), frowning as he thumbs at the edge of it. "This looks kind of deep, Kacchan. We might have to take you to Recovery Girl tomorrow."
Katsuki huffs, but stays still while Izuku cleans the bite and bandages him up. "Yeah, well, who's fault is that?" Izuku just grins. "Oh, mine, a hundred percent. I never said I wouldn't take credit for it." Katsuki snorts a laugh as Izuku presses a gentle kiss to the bandage, being careful to make sure it's nothing more than a gently brush of lips.
With a request for Katsuki to fix the blankets, Izuku goes to put the first aid kit away and grab some clean clothes for them both. Katsuki strips off the blanket that caught most of the blood that dripped, tossing it in a corner to deal with later.
He waits patiently while Izuku digs in his closet, tugging on a pair of boxers and a sleep shirt. He brings a pair of sleep pants for Katsuki, gently helping him into them. They're a little big on him, stretched to fit Izuku's thick thighs (Katsuki wouldn't mind biting into his for a change), but they fit well enough.
They snuggle under the blankets together, Katsuki's head tucked under Izuku's chin and his face buried in his neck. Izuku strokes his hair, holding him close, murmuring gentle praise Katsuki can barely hear through the sleepiness pulling him closer to unconsciousness.
The last thing Katsuki's awake for is a soft kiss being pressed to his head, a quiet "I love you." spoken into his hair before he drops off to sleep. He doesn't think he responds, but he knows he doesn't have to. He knows Izuku knows how much he loves him.
Moreso, he knows Izuku knows how much he trusts him. He would've never let him near him if he didn't, let alone in him as often as he is. (The next time will be as soon as they wake up, but they don't know that yet.)