How do I claim medical marijuana expenses on my taxes?

You can't legally do so.  Even if your state has legal medical marijuana, it is still not legal under Federal law and therefore it does not qualify as a tax deductible expense.

Tax law does not name medical marijuana specifically as a prohibited drug, especially not if it is Physician prescribed and filled by a pharmacy. I would submit that state laws proclaiming prescription marijuana as a legal prescription medication with protocols that meet a state standard is a legal and valid deduction. The IRS requirements of reasonable and necessary would support this claim. No code has specifically been written excluding medical marijuana, that in itself, lends credibility to the claim.

It would be futile to write such a code with 35 of 50 states in the process of making prescription marijuana a legal alternative pain medication.