I live in a "medical marijuana" state and, because of painful
herniated discs, am authorized to smoke marijuana in my state. If I
were arrested  in a non-medical-marijuana state for smoking marijuana,
could I successfully assert "full faith and credit" as a defense?

In theory, yes.  In practice, no way.  Even in your own state with
medicinal legality, the Feds can throw you into jail, especially if
you are an activist or attract their attention in some other way. 
This battle is currently being fought by the state of California
against the feds.  CA won some recent decisions, but the Feds have not
surrendered.

The war on drugs has too many martyrs already.  I just want you to be
safe.  Best wishes.

Thank you for your comment.   I wish to be safe, also. 

 In order to remove the fed/state prodecution issue, let me re-state
the question as "could I successfully assert "full faith and credit"
as a defense against a *state* prosecution?"  And, of course, when I
say *successfully assert a defense*,  I realize that this is probaly a
case of first impression, so no one can say with authority what would
happen, but what do you think

imho, I think if you tried it, the court would try to make an example
out of you.  If you did get arrested, and did *not* rock the boat, a
judge might be very lenient toward you, considering your
circumstances.

I want very much to see such a case successfully argued, but I am
pessimistic.  The venue and circumstances would have to be just right,
such as a "saintly" med mj smoker arrested while making a necessary
trip (like to visit a very sick relative) in another state, which also
happens to be the most liberal court of any non-med mj state....

If all the planets are aligned, it could happen, but this is all just speculation.

First, Daytrader_7, thank you for your participation. This is very interesting.
   Originally you said that in theory I could assert FF&C, but in
practice I'd lose because of federal statutory supremacy. I then
re-stated the question assuming that I was prosecuted by the offended
state -- thus no federal question. You then basically suggest it's a
matter of facts (I'm going to see a sick relative, a friendly second
state, etc.), but I'd like to keep it at the constitutional level. 
Once again, recognizing that we can't know the answer because (I
assume) this is a case of first impression, let's say I'm in any state
that doesn't have medical marijuana laws but that does recognize that,
like all states, it is subject to the FF&C clause of the US
constitution. I assert FF&C as a defense; if it is acting ethically,
how does the state rebut this defense?
   Note the similarity to the situation where gays marry in MA and
subsequently assert that FF&C requires their home state to recognize
the validity of their marriage. Conservatives appparently agree with
this argument; witness the many attempts to amend state constitutions.
   Thanks again for your, or anyone else's, participation.

There are many areas in which states do not grant legal reciprocity.
For example, a permit to carry a concealed weapon may be honored by
some states and not by others.

Good point,  pinkfreud.  So we need to ask why the brouhaha over gay
marriage?  I'd suggest that gay marriage is allegedly a form of
marriage, and the right to marry is fundamental and protected under
substantive due process. A law prohibiting same would have to meet the
strict scrutiny test: necessary to achieve a compelling state purpose.
 Perhaps the state could win that argument (perhaps not), but it's
clear that the right to smoke marijuana for medical purposes, or carry
a concealed weapon, is far less fundamental. To be sure, there *is*
governmental discrimination in both the concealed weapon and medical
marijuana cases, but the discrimination doesn't fall in the most
important categories (marriage, tracel, race, alienage, national
origin, all of which require the strict scrutiny test); or gender and
illegitimate children (which require the intermediate scrutiny test).
Rather medical marijuana and concealed weapons would fall under "all
others", which use merely a "rational basis" test, in which the
government always wins. So I think *that* is the reason FF&C would not
apply to medical marijuana.
   What do you think, daytrader_7 (and all others)?  Thank you again.

http://dictionary.law.com/definition2.asp?selected=799&bold=%7C%7C%7C%7C
full faith and credit
n. the provision in Article IV, Section 1 of the U.S. Constitution
which states: "Full faith and credit shall be given in each State to
the public acts, records and judicial proceedings of every other
state." Thus, a judgment in a lawsuit or a criminal conviction
rendered in one state shall be recognized and enforced in any other
state, so long as the original judgment was reached by due process of
law. Each state has a process for obtaining an enforceable judgment
based on a "foreign" (out-of-state) judgment.

Notice it says "judgement in lawsuit or a criminal conviction."  The
intent of this clause is to prevent the chaos that could ensue when
two states decided to feud with each other, and to ease state-to-state
extradition.  Also, it preserves the wealth of the wealthy, and keeps
the "bad guys" in jail.  Especially since many of the "bad guys" were
guilty of property crimes.

The non-med mj state would argue that to invoke FF&C would be for the
med mj state to impose its law on the non-med mj state - the concealed
carry argument is a good example of the case that would be made.

once again, imho, the Federal Goverment has been expanding its powers
so much for so long that there is no longer any such thing as states'
rights.   The constitution says that the powers not delegated to the
feds are reserved for the states.  Today, the reality is that the only
powers delegated to the states are the ones that do not interest the
Federal government - roads, schools, and most prisons.  And the states
are not even allowed to set their own speed limit on the roads that
they build and maintain.  The battle over speed limits is another
example of how the Feds dismantled states' rights long ago.

No you can't.

I believe you're missing the point of full faith and credit.  Full
faith and credit means you can't be penalized by Texas for smoking the
reefer in California.  Otherwise, you have to obey Texas laws in
Texas.