According to Rav Efraim Zalminovitch, the Chief Rabbi of Mazkeret Batya, has paskened that medical marijuana is kosher. He qualifies his ruling by stating that it is only kosher when used for medical purposes, to relieve pain, but not when used for recreational use.

I did not even realize it was a question. After all, medical marijuana is an herb, naturally grown with no ingredients that require kashrut supervision. One can debate the permissibility of certain uses of marijuana, but the kashrus of it should not be a matter of question. I'd think. And I also would have thought that the medical usage of it would not be under question, as most things are allowed when there is a medical need with no equivalent alternative.

Rav Zalminovitch himself has said in the past that giving medical marijuana to a suffering patient is a mitzva, but now he has added that the medical marijuana is kosher for medical use, though it is "treif" for recreational use. The only time it is kosher is for medical use and upon instruction from a doctor..

Rav Zalminovitch also called upon Chairman of the Knesset Yuli Edelstein and Minister of Justice Tzippi Livni to give a clear allowance for medical marijuana as an official pharmaceutical drug.

Like MK Moshe Feiglin who made medical marijuana one of his prime issues after realizing how effective it is at reducing the pain and suffering in a way that conventional drugs do not, Zalminovitch also came to the recognition after personal experience - with his ill mother.
(source: NRG)

I think "kosher" and "treif" are the wrong words to use. The correct words should probably be kept in the realm of "muttar" and "assur" - "permitted" and "prohibited". I also think the marijuana use, not abuse but use, is far less dangerous than things like alcohol - and if alcohol or smoking are allowed than marijuana use (not abuse) should definitely be allowed.