From personal experience (I have asthma as well), I've noticed it doesn't really make it that worse. On the contrary, I always find (whether I eat it or smoke it) that I am able to breathe much better. I do know that THC dilates the large vessels in the lungs as well as the air sacs, which can result in easier breathing. The vaporizer is definitely the best idea for asthma patients aside from eating it; this way you aren't inhaling so many carcinogens into the lungs, as any kind of smoke can be detrimental to them. However, a study done this year showed that marijuana doesn't appear to carry a risk for lung cancer. Other studies show that it is less likely to cause emphysema (vs. tobacco) as well. Also, if my stoned memory serves me right, marijuana used to be used as an asthma treatment before it was illegal. Hope this helps.