If someone is just going through a phase, this can be found if transiting Neptune is making strong aspects to any of their natal planets. The person will usually be off drugs when the transit goes out of orb, although this can take years.
Neptune in the first house, Neptune placed at 25 degrees of fixed signs, Neptune prominent, Neptune conjunct the ruling planet/s or co-rulers, or in close aspect to the rulers; all of these can indicate a very strong tendency to drug and/or alcohol addiction, along with other aberrations and even mental illness. Neptune conjunct the ascendant from the 12th house also factors into the above.
© Copyright 2003, 2013, Joy of Satan Ministries; Planets: Their Signs, Houses and Aspects
Neptune rules drugs and poisons. A prominent Neptune in one's chart can either be sublimated into meditation, mysticism, music and/or art, or it can produce aberrations and/or drug/alcohol addiction. There is also a relation between drug addiction and musicians, both ruled by the planet Neptune.
Library of Congress Number: 12-16457