Like knockando suggested, oral or rectal use shouldn't be a problem.
I am a serious asthmatic so the moment I saw the title of your thread I wanted to see how I could help. The ROA's that I've found have been the most triggering for my asthma have been insufflation and IV, especially with street drugs, but the same applies to pharmaceuticals. I've wound up in ICU about five times due to asthma triggered by heroin and cocaine.
For a while I thought what was happening was that it was actually the result of my DOC, heroin, which triggers the release of histamine. However, it didn't make sense, as NO pharmaceutical opiate has ever brought upon an attack, and once I got an asthma attack after a night of some serious intravenous cocaine use, I became convinced that the asthma was probably caused by a cut, not by the actual drug itself (though heroin/opiates definately can cause asthma). I'm pretty sure in my case this is true, because I can do certain batches of heroin for indefinite periods of time, and others will trigger attacks within a matter of days (in fact, this recently happened to me, though luckily I was able to catch it in time and treat it myself before I ended up in the ER).
Anyway, it is true, that for us asthmatics, insufflation, inhalation and intravenous drug use is the most risky. Oral and rectal use should be fine, and luckily, rectal almost always has a pretty decent BA, as high, or higher than insufflation.
Also, while CNS depressants can cause resperitory depression, IME, that doesn't mean that they are going to cause an asthma attack. Just be careful and don't overdue it with the benzodiazepines (certainly avoid mixing them with opiates, Alchohol or any other depressant.)
So in conclusion, avoid street drugs entirely, or if you're going to use them, try rectal administration. However, if the histamine release from drugs like opiates does cause you to have attacks, changing the ROA wouldn't make much of a difference seeing as histamine would be released no matter what. Of course it would be safer to plug or take the opiates orally as your still saving your lungs from pill binders etc, but you wouldn't avoid the histamine release. Also, I'll find the source later, but histamine release caused by opiates in the lungs for some reason does not respond effectively to common antihistamines like Diphenhydramine, Hydroxyzine, etc.. I believe it has to do with the fact that thoe drugs are H1 receptor antagonists while the histamine releases in the lungs by opiates occurs at the H4 receptor. I don't know the precise science of this, but I'll try to find the article I read this in and post it here later as I found it pretty interesting.
Anyway, asthma is a real bitch, and pretty damn scary. It's important to learn what triggers your attacks and avoid those things like the plague.
