Inspired by Toxicity on ao3. You should definitely go read it. By far one of the best one-shots about Robin I've ever read.
Edited as of 4/16/18, because there were so many errors in the original.
Once upon a time, he didn't laugh like he does now.
Not that how he laughs is bad, but it certainly isn't how he used to.
When his parents were still alive, they would tell him he had a fun laugh. The type of laugh that makes others want to laugh. He would giggle when his mom told him that, and then she would tickle him until he couldn't breath and his dad would save him. Then both of them would turn on his mom and tickle her.
After his parents died he doesn't laugh like that for a long time.
He's training with Bruce when he finally does laugh again. He's flipping and dodging the man's strikes as they spar. He stops jumping, just for a moment, and lets Bruce run at him before he slides out of the way. Bruce doesn't expect it and trips on his cape.
The sight of the great Batman laying sprawled out on the ground tangled in his cape is too much, and Robin bursts out laughing. He can't stop.
Of course, while he's distracted Bruce manages to catch him and their sparring session is cut short when they both end up laughing on the ground. It's the first time he's heard Bruce laugh.
They go upstairs and head to bed after they manage to stop laughing on the cold cave floor. As they walk up the stairs and into the Manor, Bruce bends down and looks Robin in the eyes and thanks him for helping him to laugh again.
Robin tells him he'll have to get used to it then.
When he starts to go out as Robin full time he laughs as he swings above rooftops while fighting villains, and while doing everything else. He starts to laugh out of costume again too, even though it still hurts a little knowing that his parents aren't there to laugh with him. But they wouldn't want him to stop laughing just because they died.
It's a few weeks into working full time as Robin that he meets the Joker for the first time.
The Joker laughs a lot too, but his is different. His laugh doesn't sound fun or happy. His laugh sounds insane. It sounds like pain and cruelty, wrapped up in a noise. And when he hears Robin laugh, well, it's the first time Robin wishes he didn't laugh.
Their first encounter ends quickly enough. Batman swoops in and stops the crazy clown while he's busy taunting Robin.
When the Joker's being carted away by the police he speaks to Robin with a crazy grin on his face.
"I'll be seeing you again sometime birdy. I have a feeling we'd have a laugh together!"
Robin suppresses a shudder and gets away as quickly as possible.
"Is the Joker always that creepy?" he remembers asking Batman.
The man gives a shrug. "Some days he can be worse. Are you okay?"
Robin nods, and they take off. He doesn't want to say how much that encounter unnerves him. If he does, who's to say Bruce won't decide he can't work as Robin anymore? He just hopes the clown stays locked up for a while.
Joker escapes two weeks later.
It seems like a nightmare. The news comes from Alfred while they're on patrol. Robin turns around to get back to Batman, only a few roofs away, when he feels a rough cloth cover his mouth, smells something strange, and blacks out.
Robin remembers waking up with a pounding headache and both wrists tied tightly to a chair in a dark warehouse.
He tries to find something in the dark room that'll tell him what's going on, but there's nothing. Then he tries to get free from the ropes cutting into his wrists but to no avail.
He hears a pounding noise from somewhere in the blackness and his head snaps up in the direction of the sound. Then a person with skin the color of chalk and a shock of green hair emerges from the shadows, holding a canister, like the ones that hold cheap hairspray.
"Hello Birdy," Joker says cheerfully. "How have you been? Didja miss your Uncle J? I know I missed you!"
Robin looks at Joker in disgust, but he also knows that the only way to get away from this alive will be to distract the clown until Batman can find him.
"Well, I was good until you escaped," he answers, ignoring the other two questions.
The Joker leans right up to his face and smiles a ghastly smile, skin stretching and yellow teeth showing.
"Oh, well that's not good. I wanted you to miss me. Oh well, at least now that you're visiting me we can have some fun! I brought my best batch of Gas," he says, giggling madly.
Robin stares at the canister in horror, but Joker continues.
"I liked your laugh a lot when I first met you, so I got to thinking what if we could make it better? So, I'm goin' to teach ya your Uncle J's special laugh."
"I don't know," Robin says, and wracked his brain desperately for a way to stall the Joker. "I would feel bad copying yours, after all, copycats aren't funny."
Joker paces, and for a moment Robin thinks he's stopped the madman.
"You are right birdy, copycats aren't fun. I know," he begins with renewed enthusiasm. "We could make yours special! Instead of being like mine I'll give you my secret recipe for giggles and see how much it takes for you to crack. Doesn't that sound fun!"
Robin shudders and starts to pull on the bindings holding his wrists more desperately, but before he can do anything more Joker is upon him with the Joker Gas. Robin stops struggling and takes a deep breath. The Joker starts spraying the Gas on him, but Robin holds his breath, hoping that Batman will come save him before he inhales any of the Joker Gas.
After about a minute Robin's lungs began to burn and Batman still hadn't arrived yet, so chances of him getting there in time to prevent Robin from having to inhale any of the Gas was slim to none. Robin finally can't hold it in any longer, so he stops holding his breath and begins to take deep, greedy gulps of air. It's only when his lungs begin to tickle that he remembers why he was holding his breath.
He takes one last deep breath, but because he's already inhaled some Joker Gas he can't help but giggle, just a little bit. Then he starts to laugh, and then guffaw. By that time he's struggling to breath, the laughter hitting him too hard.
And the scary part is that he can't stop.
He's aware that Joker is still spraying Gas into his face, and somewhere in the back of his mind, he wonders if inhaling this much Joker Gas will have negative effects on his health. Or if the antidote they have will even be effective.
Then he feels Joker shove the canister of Gas closer to his nose, effectively causing him to inhale even more, and he starts to cackle and laughs harder. He can't stop laughing, and at this point, he's not quite sure if he wants to. And that realization is terrifying, more so than anything else.
Eventually the urge to laugh stops getting worse, it doesn't get better, but it stops getting worse. He wonders vaguely why he hasn't passed into a coma yet, like most others who inhale even half as much Joker Gas as he has.
Then, through the haze of laughter, he hears the Joker, sounding faint and far away.
"Now, that's more like it birdy! I like this laugh a lot better. I wonder what'll happen if I give you more!"
Something presses up against his nose again, and he starts to laugh even harder, well, at this point, it sounds more like a cackle. Time starts to lose meaning, and so do all his other senses. He only becomes aware that there isn't anything making him laugh harder when he feels himself being jostled around like someone's picked him up.
He passes in and out of awareness, but after a while, the feeling that he needs to laugh starts to lessen. He can't help but feel disappointed at first. Before he can think anything more he passes out.
When he wakes up, a sense of fear grips him and he immediately shoots up, looking around for danger, only to see Bruce asleep in a chair. It only takes a second longer for him to realize that he's on one of their medical tables.
Was that a dream? he finds himself wondering.
He tries to sit up, but it doesn't quite work, and he feels a pain in his lungs before he collapses back onto the bed. Now that he thinks about it, his throat is sore too. In fact, he feels like he has one of those colds that leaves you coughing all day and night.
"Dick!" Bruce's voice exclaims from his side. "You're awake. How are you feeling?"
Dick turns his head to look at Bruce and gives a weak smile. "Like I've been hit by a truck. What happened?"
Bruce looks at Dick worriedly. "What do you remember?"
Dick thinks back, and all of a sudden the warehouse comes back to him. "I was grabbed by the Joker right after Alfred warned us about him escaping Arkham. He tied me to a chair and sprayed my face with a ton of Joker Gas. Like, more than I've ever seen anyone inhale."
"That's impossible Dick. The amount you're suggesting would kill someone almost instantly after they began laughing. Or make them go insane."
"Oh," Dick says with wide eyes. "Well, I don't think I'm in a coma, or dead. Pretty sure I'm not crazy. Although if I was, that's probably what I would say to make you think I'm not. But if I was crazy I wouldn't admit that. I mean, I don't think I would. I'm not crazy, I prom-"
"Dick," Bruce says exasperatedly. "I don't think you're crazy. You must not have breathed as much in as you think"
Dick isn't quite sure about that, but he's not about to argue. Still, he'd rather be sure.
"Bruce," he begins. "I am pretty sure that he was spraying the Gas at my nose the whole time, and I'd rather be safe than sorry."
"You want me to check your blood?" Bruce asks.
"Well, I do know you well enough to know that you would have taken a before the antidote sample," Dick says with a grin.
Dick watches as Bruce moves over to the Batcomputer and watches as Bruce begins to look at a complicated series of numbers. A moment later he stiffens, and then he looks right at Dick.
Dick narrows his eyes. "What's wrong Bruce?"
"Joker gave you, a lot of Joker Venom."
"And…"
"And there might be side effects," Bruce says without looking at him.
Dick takes a moment to process.
"What do you mean side effects? Like, I really could go crazy?"
"No. I don't know. But if you feel any different than you normally do, then you need to tell me or Alfred," Bruce said.
Dick nods. He knows that ignoring a problem will just make it worse. Both of them do.
Nothing seems to be wrong for a while. He heals just like he normally does. He doesn't act any different, at least that he notices. Really though, he knows nothing can ever end well when the Joker's involved.
He's doing the normal type of thing he does on weekends. Playing video games and eating unhealthy food that Alfred would kill him for even possessing.
Then, in his video game, a character falls off the edge of a cliff and Dick bursts out laughing, and it feels so good. Like he's been missing it, and an invisible pressure he hadn't even know is there has been lifted off of him.
He stops and claps his hand over his mouth in horror. His laugh, that isn't normal. It doesn't sound normal, and while he does like to laugh, it feeling like a fun thing to do and good stress relief isn't normal. Anything like this developing after being with the Joker definitely isn't good.
So Dick runs downstairs with his hand still clamped over his mouth and seeks out Alfred. Once he finds the old butler dusting he signals that he needs Bruce, and luckily Alfred seems to get it.
"Master Bruce is in his office."
Dick takes off running and bursts into Bruce's office, much to the man's surprise. The older man sees the panicked look on his face and comes over, asking "What is it?"
Dick shakes his head, then carefully takes his hand off of his mouth. When he doesn't get any urge to laugh or smile he starts to talk.
"I started to laugh, and I know that how it sounds now is not how it sounded a week ago," Dick says quickly. He wants to finish as soon as possible just in case he starts to laugh again.
"Okay," Bruce says, running a hand through his hair. "Let's get down to the cave and check your blood for Joker Gas."
Dick nods in agreement.
They go down into the cave. Thirty minutes and a few ounces of blood later, the results came back.
Bruce reads the computer screen and then frowns in Dick's direction.
"What's wrong?" Dick demands.
Bruce's face is furrowed in concentration. "It looks like you have trace amounts of Joker Gas still in your system."
"What does that mean?" Dick asks. "Shouldn't the antidote have gotten rid of it? And I don't feel the need to randomly laugh."
Well, that was kind of untrue, but it's a controllable need.
"It should have, but it's possible that Joker was using a slightly different formula. Is it only your laugh?" Bruce asks, ignoring Dick's last statement.
Dick hesitates a moment before answering. "Yeah, it's only my laugh."
It may be dangerous, but what Bruce doesn't know won't hurt him. And if he did know that all of a sudden laughter sounds like a fun thing to just do for no reason, or that it's like a weight on his lungs when he doesn't laugh, well, Dick can kiss Robin goodbye.
He can deal with this.