(9) Only take heed to thyself, and keep thy soul diligently, lest thou forget the things which thine eyes saw, and lest they depart from thy heart all the days of thy life; but make them known unto thy children and thy children’s children;
(טו) וְנִשְׁמַרְתֶּ֥ם מְאֹ֖ד לְנַפְשֹׁתֵיכֶ֑ם כִּ֣י לֹ֤א רְאִיתֶם֙ כָּל־תְּמוּנָ֔ה בְּי֗וֹם דִּבֶּ֨ר יְהוָ֧ה אֲלֵיכֶ֛ם בְּחֹרֵ֖ב מִתּ֥וֹךְ הָאֵֽשׁ׃
(ו) שֹׁמֵ֣ר פְּתָאיִ֣ם דַּ֝לּוֹתִ֗י וְלִ֣י יְהוֹשִֽׁיעַ׃
Maimonides' legal code is called Yad ha-lfazakah (the Strong Hand). In the section dealing with "Murder and the Guarding of Life" he writes:
It is a positive commandment to remove any stumbling block which constitutes a danger and to be on guard against it. The sages have prohibited many things because they endanger one's life. If one disregards any of them and says "I am only endangering myself, what business do others have with me; or I don't care [if they are dangerous] I use them (that is, harmful things)," he can be subjected to disciplinary flogging.
(כ) ראובן חפר גומא בחצירו שמי גשמים יקלחו לתוכה וכשרבו המים בוקעין ועוברים דרך חומות מרתף שמעון חייב ראובן לסלק היזקו: הגה: וכל היזקות שלא נתבארו שיעור הרחקתן בכדי שלא יזיק לפי ראות הבקיאין (טור בשם הרא"ש):
Reuven was digging some holes in his yard,
when suddenly he created a hole that allowed all this leftover rain water to spill
through the wall, into his basement. Reuven
is halachically obligated to take care of the
damages now. (Or something like that - Aramaic aint easy to translate.)
Igros Moshe, Yoreh De’ah Vol. 3, Siman 35. The following is a loose translation of what he writes there:
“It is obviously forbidden to smoke marijuana, as this violates many basic laws of our Torah. First of all, it physically injures the person. Even if there are people who are not physically affected by this, it mentally affects the person as it destroys his mind, and prevents him from understanding things properly. This is a terrible thing, since not only can the individual not properly study Torah, he also can not pray and properly perform Mitzvos (commandments), since doing them mindlessly is considered as if they were not done at all. Furthermore, he is creating within himself a very strong desire (addiction?), which is much stronger than the desire to eat, etc. which are necessary for a person to live. There are many that can not control and withstand this desire. This is a very grave prohibition, as we find that a Ben Sorer U’Moreh [is killed] (See Deut. 21:18) for creating within himself a very strong desire, even though it is to eat Kosher food! How much more so it is forbidden for a person to bring upon himself an even greater desire, especially for something that a person does not need at all…"
The thing is, for a long time, Jews smoked quite a bit like everyone else did. Until scientific journals started to come out and speak of the harsh effects of smoking, many Rabbis smoked regularly. Without knowing the true effects of smoking, it remained a halachic gray area until the science started to trickle into common knowledge.
Unfortunately, smoking still remains a very common practice throughout Israel, with many Chareidim in Mea Shearim smoking as well. Here is a picture from Purim 2015, courtesy of Vice.
Another very interesting source that proves the unknown of the effects of drugs and smoking comes from the Shulchan Aruch talking about Shabbis Candles:
"Also, one will beautify [Shabbat candle lighting] when the wick is made from cotton, flax or cannabis…"