One grain is equal to just under 65 milligrams.
It is difficult to answer your question about v iss without knowing more information. There are several medical meanings for this.
my problem is seconal gr iss po bedtime prn
available is seconal 100 mg capsules. how many capsules will you give?
grains is a measurement from the old apothacary system. The previous cburr is correct that one grain is approx. 65 milligrams. A common prescription we used to see was ASA v gr. this meant asprin (ASA is the abbreviation for the chemical name of aspirin) 5 grains (325mg-one regular strength asprin tablet). so v iss was probably vi ss. ss = 1/2 in medical speak. this would be 6 1/2 grains, or 422mg. --probably rounded to 425mg.
hope that helps