There was an excellent post on this question on 1/5/2008
http://www.medhelp.org/posts/show/420879
There have been other questions about marijauna posted on this site you may want to look over past comments too. I personally would not recommend smoking it because I don't think a person is in complete control of all their faculties. Meaning will it impair your judgement-will you remember if you tested your blood sugar or gave yourself an injection. Will you be able to tell if you are having a low blood sugar reaction and effectively take care of it. Will it make you hungry or tired and not be able to decipher if you need some glucose or if it is just a side effect of the drug.
Now by negative effects you mean will it have long term effects. Well diabetics already have less than perfect circulation, so smoking is not good, so smoking an unfiltered drug sounds like it would have negative effects as well. Other negative effects I don't know, but look in the archived questions as I mentioned above--I remember reading something relating to side effects.