I take Methycobalamin (Vitamin B12) supplements. I noticed the tablet packaging reads "Each Sugar coated tablet contains Mecobalamin JP 500 mcg" What does JP mean here? Thank you.
It stands for Japanese Pharmacopoeia. In other places you may see USP for the US and EP for Europe.
It indicates that the substance is prepared according to the standards of that pharmacopoeia, sort of a quality control level that a drug contains the actual chemical substance it claims to contain.
It doesn't necessarily mean the drug or supplement does what a manufacturer claims, and the pharmacopoeia itself hasn't done any independent verification that the drug is what the manufacturer claims. You can consider "JP" to be shorthand for "we are claiming that the contents of this product are tested according to the standards of the Japanese Pharmacopoeia" and there could be potential legal consequences if it turns out the product does not meet those standards.