Check the general rule for bringing something into Japan by post or courier for personal use. Pick the item type; for supplements, tick any of the listed ingredients. This is general information, not legal advice, and the rules change, so always open the official source.
How to read the result
Green means it is generally fine within the personal-use amount. Amber means there is a quantity limit or a mark or condition to check. Red means it is commonly blocked or refused, for example by iHerb, or needs an MHLW confirmation. These are general guides for personal quantities, not a customs ruling. Larger amounts, resale, or anything unusual can change the answer, so confirm with the official source before you order.