I read somewhere that the shingles virus interferes with the nerve's morphine receptors and hence morphine (and derivative drugs) aren't very effective pain relief. I can attest to having been given several doses of morphine at my first ER visit for (later to find out) shingles and the relief was both incomplete and very very short-lived.
Conversely, I read somewhere else that cannabis receptors are left intact in the nerve and, following that theory, pot smoking should relieve the neuralgia (nerve pain) brought on by shingles. After further reading, it turns out there are different varieties of medicinal marijuana (who knew?!) and getting the right relief means getting the right variety.
In the case of shingles or postherpetic neuralgia, the applicable strain appears to be cannabis indica. This strain is apparently sometimes referred to as night-time weed because it allegedly has a more relaxing, sedative effect - a body buzz instead of a high, if you will.
Cannabis indica also reportedly helps relax bladder muscles and stimulate the bowels, side effects that may be helpful in the treatment of neurogenic bladder or neurogenic bowel complications of the shingles virus.