Medical doctors actually can only show causation of the danagers of smoking. They have yet to prove causation (since there are so many chemical and interactions occurring over time in cells).
Cancer on the other hand, typically hard to pull off at that level. Cancers, for the most part, are more of a "if exposed to X, cancer develops" rather then being actually caused by the continents itself (which is different then cancers caused by radioactive exposure). Cancers are a different story with smoking...but it is clearly evident some reasons the dangers of smoking that are easily proven.
But anyways, I'm going off-topic.