You might be looking for the episode, The Love Potion.
Love Potion is a very complicated potion. Put a bit of DNA inside and the drinker will fall in love with the person whose DNA is in the potion. One millitre is already enough, a bit more, than it will become an overdose. An overdose will cause the drinker to be overly obsessed and do drastic things. The potion contains hair of a Harpy which induces infatuation.