Constitutionality is irrelevant. What is RIGHT, however, is.
Not only is it your right to nullify bad statute, it is a duty you hold to yourself, as well as your fellows.
The question becomes, what differentiates mere statute from LAW?
LAW is valid because it bases in principle, whereas mere statute is nothing more than the whim of some randomly chosen legislative body.
Violation of LAW is a CRIME, whereas violation of statute that does not rise to the standard of LAW, does not. It is a simple thing, and while at first may seem difficult, with some practice becomes easier and easier, as one's habits of thought and analysis become well tuned to the task of making the determination.
Is the simple possession of methamphetamine a crime? No. Why? Because in order for a crime to have been committed, there must be a credibly demonstrated victim.
Is murder a crime? Clearly yes. Why? Because there is a victim whose property (their life) has been stolen from them AGAINST THEIR WILL.
Is suicide a crime? No. Why? Because there is no victim, even though someone loses their life. Why is there no victim? Because the act was committed with the injured party's consent to said injury. In other words, a man cannot criminally victimize himself. It is by definition an impossibility.
Given all this, you now have most of what you need to be able to make these sorts of decisions. The last thing you should know, however, is that you may not always like the answers that the truth brings to your questions on such issues. For example, we may not like that people destroy their lives with drugs such as meth. What decent man would think this a good thing? But that does not mean we choose to agree with drug laws simply because they impose upon others that which we would have imposed upon them no matter that others might think. This is a very common error on the part of well meaning people.
Never violate the freedom of a man's prerogatives to choose just because he chooses something disagreeable to you. This is a deep and cardinal sin against your fellows, no matter how noble your intentions.
Freedom requires much of the individual, including courage, smarts, intelligence, and generosity of spirit toward one's fellows, ESPECIALLY when it is most trying to manifest these qualities in the face of the grossly disagreeable. Those are the times when a man's true cloth are made most clear. It is rarely easy to be free, but the rewards are well worth the costs, IMO.
So if you feel the drug laws in question are not righteous, then it is your duty as a free and self-respecting man to nullify them and to convince your fellow jury members to follow suit.
Good luck with it.