Anything is potentially carcinogenic, if it is "foreign matter" it can cause cancer. When I say cause cancer, I mean potentially, not that it will happen in your lifetime. When I mean potentially I mean for example with an enormous dose in mice a certain product can cause cancer cells. This has been shown for example with aspartame (the sugarless sweetner in many products, namely chewing gum), it has never "knowingly" caused cancer in humans, but in mice it causes cancer in very high doses. Smoking weed can generally be regarded as not carcenogenic because of many factors, mainly that even chronic smokers smoke very little compared to the ammount that would potentially cause cancer, but the fact is that the vaporized tar contained in the plant matter is carcinogenic. Recent research also shows some link between THC and the inhibition of cancer, so most likely it is near impossible to get cancer from smoking weed alone but the fact of smoking anything is not "healthy". Smoking hash if done without inhaling butane vapors should therefor be the least harmful for you next to vaporizing.