while it's not ideal to inhale any type of smoke into the lungs, there is a key difference between cannabis smoke and tobacco smoke. the nature of tobacco smoke causes the bronchial airways to constrict, making it difficult for the lungs to expunge the unhealthy matter (carcinogens). Whether MJ smoke has more or less carcinogens than tobacco is pretty irrelevant; MJ smoke causes the bronchial airways to DILATE, making it easier for the lungs to cleanse themselves. It expedites the process if you exercise a lot. I haven't perused the links provided by Storm Cow, but i bet there's something in there about how that specific property of cannabis smoke decreases cancer risk and curtails the process by which malignant cancer cells are nourished.

In short, be moderate, smoke only the best, exercise frequently, and let your body be its own gauge of whether what is being imbibed is healthy or not. Some people have a hard time using cannabis and functioning normally, but to others it can be monumentally helpful.