Srila Prabhupada advises a confused disciple in a letter about the use of drugs.
You have asked about "chemical changes of consciousness'' or drugs, and if these may ever be utilized in Krishna's service. To change consciousness, therefore the Spiritual Master is there. Do not do anything whimsically, like taking these drugs, that will ruin everything. Because you are imperfect you require help from the Perfect, and anything material like drugs is imperfect, so do not be distracted by contemplating in this way. This chemical is intoxicant, it will not help, it will send you to hell. So if you have received initiation, then you have got the instruction no intoxication. So why do you ask this? That means that you have still addiction to these intoxicants. It was said, and you promised, no intoxication, so why do you think of this?
(Letter to: Visvambhara — Ahmedabad 14 December, 1972)