This prohibition of the use of opium is reiterated by Bahá’u’lláh in the final paragraph of the Kitáb-i-Aqdas. In this connection, Shoghi Effendi stated that one of the requirements for "a chaste and holy life" is "total abstinence ... from opium, and from similar habit-forming drugs". Heroin, hashish and other derivatives of cannabis such as marijuana, as well as hallucinogenic agents such as LSD, peyote and similar substances, are regarded as falling under this prohibition.

Bahá’u’lláh, The Kitab-i-Aqdas, p. 238

. . . the Universal House of Justice has stated that a Bahá’í is permitted to use marijuana when it is specifically prescribed by a medical doctor.

Universal House of Justice to an individual believer, 16 December 2008

Although we have found no direct reference to marijuana in the Bahá’í writings, since this substance is derived from what is considered to be a milder form of cannabis, the species used to produce hashish, we can share with you a translation from the Persian of a Tablet of ‘Abdu’l-Bahá on hashish: “Regarding hashish you had pointed out that some Persians have become habituated to its use. Gracious God! This is the worst of all intoxicants, and its prohibition is explicitly revealed. Its use causeth the disintegration of thought and the complete torpor of the soul. How could anyone seek this fruit of the infernal tree, and by partaking of it, be led to exemplify the qualities of a monster? How could one use this forbidden drug, and thus deprive himself of the blessings of the All-Merciful?”

Universal House of Justice, Lights of Guidance, p. 353

Bahá’ís are required to be obedient to the provisions of both civil law and Bahá’í law. As regards the Bahá’í position, it is that a believer is only permitted to use substances such as marijuana when they are prescribed for medicinal purposes. However, a Bahá’í must also satisfy civil law requirements; your letter indicates that the use of marijuana is illegal in ... for any purpose, in which case it would not be permissible for a Bahá’í to use it even to meet a medical need.

Universal House of Justice to an individual, 19 September 2000

In reply to your letter of March 23, 1976, the Universal House of Justice instructs us to say that the word "marijuana" does not appear as such in our Sacred Scriptures. However, marijuana contains the same narcotic as hashish (hemp) and the same prohibition applies to its use as applies to the use of hashish.

Universal House of Justice, NSA USA - Developing Distinctive Bahá’í Communities

In reply to your request of October 24, 1967 that we issue a statement concerning 'the use of marijuana, LSD and other psychedelic products', we have already informed the National Spiritual assembly of the United States that Bahá’ís should not use hallucinogenic agents, including LSD, peyote and similar substances, except when prescribed for medical treatment. Neither should they become involved in experiments with such substances.

Universal House of Justice, Lights of Guidance, p. 353

The National Spiritual Assembly, at a recent meeting, consulted on the topic of medical marijuana and decided that it is permissible for a Bahá’í to use marijuana for medical purposes provided that a qualified physician has determined that it is the best treatment of the medical condition, that its use is legal in the state where the individual resides, and that the use is in private.

NSA-USA to an individual, 20 November 2015