Just wondering if I could have been building illegally huge muscles this whole time.
No. Asthma steroids= corticosteroids, used to reduce inflammation, etc. Athletes use anabolic steroids, used to build muscle.
oh, bummer. but wait. do corticosteroids also have negatives on fertility?
This is flat-out wrong.
There have been multiple cases of athletes in cycling using/abusing these drugs.
(Cris Froome is currently booed through France because he sometimes has really, really bad astma and naturally very high level of cortisoides: or whatever bullshit team Sky is spinning)
Given that they’re used in some cases involving infertility (where immune system problems are suspected) probably not.
Illegally huge muscles?
That sounds like a dystopian law if I ever heard one. 
If athletes are using corticosteroids then it’s not for building muscle because that’s not what they do. If they are using them, I’d guess it would be in an effort to increase lung capacity or some such (which would be consistent with something like the Tour de France).
If you’re on a steroid maintenance inhaler for your asthma then the corticosteroid will be mild and low dose, and won’t have any particular performance enhancing qualities. If you’re on something stronger - prednisone, for example, is commonly used for more serious asthma issues, then this is a frequently abused performance enhancing drug. As noted, it lowers inflammation and may also help with weight loss [other powerful corticosteroids do].
Drug testing actually takes place all the way down to amateur levels of sport (at least for those sports that take PEDs seriously), so if you were unaware of this and turned up to a time trial or a triathlon after a prednisone puff in the morning, say, it’s possible you could be tested and banned. This sounds faintly ridiculous, banning the athletic equivalent of man-in-pub for being unaware of drug-testing regimes, but it has happened many times. At least a proportion of those will have been people trying to cheat at what is just a small step up from recreational sport.
But corticosteroids are not going to improve lung capacity in anyone who does not have lung inflammation. Most athletes do not have that. Most athletes who use ‘steroids’ are using the anabolic steroids, as you noted. There’s a lot written about athletes and anabolic steroid abuse in the scientific literature.
There’s not a lot written in the literature about athletes abusing corticosteroids like prednisone. At least not much that Google Scholar would turn up for me. Most likely because it really doesn’t enhance performance much at all. Problems with weight gain, fluid retention, sleep disturbances tend to quickly outweigh any small benefit from inflammation reduction. Androgen-like steroids are much more potent and much more abused.
Ha. Ha ha. No, really, I’m guffawing here –
Weight gain is one of the most common and annoying side of effects of predisone. And not muscle, either, it’s fluids and fat. There is no way prednisone is going to help you lose weight.
I know that, and of course YOU know that, but that doesn’t mean other people know that.
I had an elderly cat who was getting skinny (the vet thought she was losing her sense of smell), who was put on prednisone. She gained like two pounds (to go from 6 to 8, after weighing about 9 most of her life), and then went off of it. Started to lose the weight again, and it took a while, but we finally found her maintenance dose. That and some canned catfood, and she lived to be 18.
Yeah. I was on an inhaler once in the Army, in training, after trying to tough out an infection that turned out to be bacterial, and became something really nasty (yes, I learned my lesson). So, we had a PT test, and there was another soldier who kept begging to take a hit off of my inhaler, because she was convinced it would help her pass the test. I had to hang around the DSs to get her to leave me alone. All I needed was someone else’s germs on my inhaler.
She failed the test (she was overweight anyway), but guess whose fault she was convinced it was. Nevermind, I passed and was out of there.