Dyeing one's hair via ingestible medicine

Hi,

There is a biblical prohibition against grooming one's self in the manner of the opposite gender, and this is understood to include a ban against men dyeing their hair to conceal the grayness of aging. [There are exceptions to this rule, which are beyond the scope of this post. - MT]

This ban includes ingesting a medicine to dye one's hair; the prohibition is not against the technique of dyeing, but against the result of changing the colour.

(Rav Moshe Feinstein, Igrot Moshe Yoreh Deah 1:82)

Have a great day,
Mordechai