Jennifer Lien wasn't fired because of drugs. She was fired because those in charge wanted a new character, a "sexy babe" to boost the ratings.
When they brought in Seven Of Nine in the beginning of season 4, they thought that they couldn't afford ten main characters so one of the original characters had to go. The first choice was Garret Wang (Kim) but when an inside poll among the staff of a glossy magazine showed him among "the 50 most handsome", they changed their minds and dumped Lien instead.
There was a drug rumor about a Star Trek actor (not Lien) before "the changes in season four" but that rumor was never confirmed and that actor wasn't fired either.