"Khamr is what befogs the mind." These are the words spoken by 'Umar ibn al-Khattab from the pulpit of the Prophet (peace be on him), providing us with a decisive criterion for defining what falls under the prohibited category of khamr. There remains then no room for doubts and questions: any substance which has the effect of befogging or clouding the mind, impairing its faculties of thought, perception, and discernment is prohibited by Allah and His Messenger (peace be on him) until the Day of Resurrection.
Drugs such as marijuana, cocaine, opium, and the like are definitely included in the prohibited category of khamr. It is well known that the use of such drugs affects the sensory perceptions, making what is near seem distant and what is distant seem near; that their use produces illusions and hallucinations, so that the real seems to disappear and what is imaginary appears to be real; and that drug usage in general impairs the faculty of reasoning and decision-making. Such drugs are taken as a means of escape from the inner reality of one's feelings and the outer realities of life and religion into the realm of fantasy and imagination. Added to this psychological fact are the physical effects: bodily lassitude, dullness of the nerves, and decline in overall health.
The moral consequences, moral insensitivity, weakening of the will-power, and neglect of responsibilities are also well known. Eventually, addiction to drugs renders a person a diseased member of society. Furthermore, drug addiction may result in the destruction of the family or even in a life of crime. Since obtaining drugs involves a great outlay of money, a drug addict may well deprive his family of necessities in order to buy drugs and may resort to illegal means to pay for them.
When we recall the principle that impure and harmful things have been made haram, there can be no doubt in our minds concerning the prohibition of such detestable substances such as drugs, which cause so much physical, psychological, moral, social and economic harm.
The Muslim jurists were unanimous in prohibiting those drugs which were found during their respective times and places. Foremost among them was Sheikh al-Islam Ibn Taymiyyah, who said, This solid grass (hashish) is haram, whether or not it produces intoxication. Sinful people smoke it because they find it produces rapture and delight, an effect similar to drunkenness. While wine makes the one who drinks it active and quarrelsome, hashish produces dullness and lethargy; furthermore, smoking it disturbs the mind and temperament, excites sexual desire, and leads to shameless promiscuity, and these are greater evils than those caused by drinking. The use of it has spread among the people after the coming of the Tartars. The hadd punishment (The Qur'an specifies the punishments for certain crimes, such as lashing for drinking wine and equal retaliation or compensation in the case of murder or injuries. These punishments are called hadd (plural, hudud), meaning "the limit set by Allah." (Trans.)) for smoking hashish, whether a small or large amount of it, is the same as that for drinking wine, that is, eighty or forty lashes.
He explained the imposition of hadd for smoking hashish in the following manner: It is the rule of the Islamic Shari'ah that any prohibited thing which is desired by people, such as wine and illicit sexual relations, is to be punished by imposing hadd, while the violation of a prohibited thing which is not desired, such as (eating) the flesh of a dead animal, calls for ta'zir. (For crimes concerning which no specified punishment is mentioned in the Qur'an or Ahadith, the Muslim government may introduce its own punishments, such as fines or imprisonment. Such a punishment is called ta'zir. (Trans.)) Now hashish is something which is desired, and it is hard for the addict to renounce it Accordingly, the application of the texts of the Qur'an and Sunnah to hashish is similar to that of wine. (Fatawa Ibn Taymiyyah, vol. 4, p. 262 f. Also see his book, Al-Siyasah al-Shar'iyyah.)