By the Grace of G‑d,
20th of Marcheshvan, 5725
[Oct. 26, 1964] Brooklyn, N.Y.Greeting and Blessings:
I am in receipt of your letter of October 18th, which you write in the name of your friends and in your own behalf, and ask my opinion regarding the new drug called L.S.D., which is said to have the property of mental stimulation, etc.
Biochemistry is not my field, and I cannot express an opinion on the drug you mention, especially as it is still new. However what I can say is that the claim that the said drug can stimulate mystical insight, etc., is not the proper way to attain mystical inspiration, even if it had such a property. The Jewish way is to go from strength to strength, not by means of drugs and other artificial stimulants, which have a place only if they are necessary for the physical health, in accordance with the Mitzva to take care of one’s health. I hope that everyone will agree that before any drugs are taken one should first utilize all one’s natural capacities, and when this is done truly and fully, I do not think there will be a need to look for artificial stimulants.
I trust that you and your group, in view of your Yeshiva background, have regular appointed times for the study of Torah, and the inner aspects of the Torah, namely the teachings of Chassidus, and that such study is in accordance with the principle of our Sages, namely “The essential thing is the deed,” i.e., the actual conduct of the daily life in accordance with the Torah and Mitzvoth, prayer, Teflllin, Kashruth, etc., etc. This is only a matter of will and determination, for nothing stands in the way of the will. I trust that you are also using your good influence throughout your environment.
With blessing,
Source: Chabad.org here.