Is it permissible to disregard the prescribed uses written on the medicine, and is that considered a betrayal of trust?

The basic rule is that it is permissible to use medicine for whoever possesses it if its benefit is overwhelmingly probable. It is forbidden to contravene the leaflet containing the instructions for use if doing so results in harm, based on the Prophet's (peace be upon him) saying: "There shall be no harm or reciprocating harm." It is disliked to use it if there is no need and no harm results from it, because that is placing something in other than its proper place and involves extravagance.