I know it's discouraged for Buddhists to intoxicate their body, but were there any known cases of monks using psychedelic drugs intentionally in the past?
Buddhism is about having in-body experiences not out-of-body experiences. Psychedelics distort one's experience/perspective of reality and Buddhism is about experiencing reality for what it really is. Thus psychedelic drugs are the opposite of the dharma path and not used by monks or nuns.
The short answer is "yes, historically, though it is generally frowned upon by many if not most modern Buddhists." I'm trying to find better citations than the ones than I am digging up immediately, but the best sources for this don't appear to be readily available online (or require more reading through to figure out what is being referred to). If anyone has some better references I'd love to see them, since mostly what I am finding are three or four pieces that reference one another.
Regardless, moving on to the actual answer: In older texts you see a lot of references to what is probably Datura (certainly in some cases, likely in others) in early Buddhist magical texts and in a lot of precursors/early forms of Vajrayana. Ronald M. Davidson's Indian Esoteric Buddhism, for example:
Moreover, many of the siddha scriptures discuss ointments and drugs, especially those applied to the eyes or feet. The use of the various species of datura (especially datura fastuosa [Datura metel]) is particularly evident.
This is most likely used for the entheogenic use. I've found some other references along these lines but I want to chase down a translation of the original rather than depending on a secondary sources before posting it here.
Depending on your view of the Saṁvarodaya-tantra it describes the use of datura (which is translated "thorn apple"–from sanskrit dhatūr, which is where we get the word datura–by Shinʹichi Tsuda in The Saṁvarodaya-tantra: Selected Chapters), but not really the imbibing of it, as far as I can tell, just the use for magical purposes. There are quite a few of these and similar references in older Indian sources, see for example Datura Rituals in the Vajramahabhairava-Tantra. I wouldn't necessarily take the examples given, however, as indicating entheogenic uses as opposed to, say, sympathetic ones.
There are also numerous examples of–particularly Western–individuals who have experimented with this, see for example Zig Zag Zen, though I don't know that any of the references described in that book are of monks (I haven't read it, just found it while researching).
It wouldn't surprise me if, once one does some digging, one can find some sect of Buddhism somewhere that uses or used datura and/or cannabis for entheogenic reasons. There are sources that indicate that this is not even controversial, and while at the moment I am having difficulty tracking some of them down, there are certainly plenty of examples with the consumption of alcohol.
This doesn't mean it is widely accepted or a good practice, however, and as the expression goes: "you drink because of the ritual, you don't go to the ritual to drink."
Psychedelic is an intoxicant leading to heedlessness hence not acceptable in Buddhism even for lay people. When it comes to monks who have a higher standers of Sila this is certainly not acceptable. I don't think monks would have used such substance at any point in history without breaking the Vinaya.
This is well explained in "THE FIVE PRECEPTS" which is a very good read on the 5 precepts.
The fifth precept reads: Surā-meraya-majja-pamāda-ṭṭhānā veramaṇī sikkhāpadaṃ samādiyāmi, “I undertake the training rule to abstain from fermented and distilled intoxicants which are the basis for heedlessness.” The word meraya means fermented liquors, sura liquors which have been distilled to increase their strength and flavor. The world majja, meaning an intoxicant, can be related to the rest of the passage either as qualified by sura,meraya or as additional to them. In the former case the whole phrase means fermented and distilled liquors which are intoxicants, in the latter it means fermented and distilled liquors and other intoxicants. If this second reading is adopted the precept would explicitly include intoxicating drugs used non-medicinally, such as the opiates, hemp, and psychedelics. But even on the first reading the precept implicitly proscribes these drugs by way of its guiding purpose, which is to prevent heedlessness caused by the taking of intoxicating substances.
Siddhartha and Jesus of Nazareth are always quoted but the citations to their authorship is really reality vague! Tantric vs. Non-Tantric, i know the way and that's not it because Drama told me so and on and on it goes....... 1. Learn to extinguish your ego 2. Work, Compassion, Knowledge all the well springs of human nature... drink from these wells. 3.Cultivate Stillness as a practice 4. Lastly: " When it comes your time to die, be not like those whose hearts are filled with the fear, so that when their time comes they weep and pray for a little more time to live their lives over again in a different way. Sing your death song and die like a hero going home." Tecumseh, Shawnee Warrior,Leader, and no doubt a Buddha. Oh Yea i almost forgot Don't forget to laugh... especially at your self. ROGER THAT>OVER&OUT. re: Psychedelics can help but be very careful...they are not the way only a map reference. Ayahuasca may help somewhat. Good Luck on your journey.
Taking drugs is does not provide any clarity only more maya or illusion.