Praise be to Allah.
Marijuana is an intoxicating substance; it is derived from the cannabis plant (Cannabis indica), which is a plant that has an intoxicating effect. Based on that, the ruling on it is that it is haraam (prohibited), as has previously been stated in detail and with evidence in fatwa no. 176545
With regard to mescaline, it is a substance derived from a type of cactus that is known in north-eastern Mexico as peyote. It is an intoxicating compound which causes hallucinations and colourful visions.
Psilocybin is a substance derived from some mushrooms, such as the Mexican psilocybin mushroom. This is also an intoxicating substance, with a stronger effect than mescaline
With regard to what you mentioned about LSD, this is another name for lysergic acid diethylamide. It was first synthesized in 1938 by a Swiss pharmacist called Albert Hofmann. It is taken in micrograms because it is one of the strongest of all hallucinogenic drugs. This drug was banned in 1966 and was classed as an (illegal) drug because of the negative consequences of taking it, such as suicide and so on.
What is mentioned in the question about these drugs having no harmful effects is to be rejected. In fact they cause serious harm, both psychological and physical. That is because psilocybin, mescaline and LSD are all hallucinogenic, intoxicating substances that have a great impact on thinking, mood and behaviour. They lead to a lot of illusions, mental confusion, and aural and visual hallucinations, as well as physical lethargy, schizophrenia, and changes in perception. The one who consumes this substance may have fits of laughter without any reason.
The symptoms experienced by the one who consumes these drugs may also include loss of a proper sense of time and place, and altered perception thereof. He may imagine a minute as lasting for a lifetime, or he may see stationary objects as if they are coming closer to him or moving further away from him.
This is in addition to some physical symptoms, such as nausea, excessive burping, sweating, paleness of the face, dilation of the pupils, rapid heartbeat, and so on.
Based on that, these substances are haraam because of this far-reaching physical harm. The Prophet (blessings and peace of Allah be upon him) said: “There should be no harm nor reciprocating of harm.” Narrated by Ahmad in his Musnad (2865), Ibn Maajah (2341); classed as saheeh by al-Albaani in Saheeh Ibn Maajah
If we assume, for the sake of argument, that the negative impact on health of these drugs was not known, it would still be sufficient – in order to disallow them – to know that they are classed under the heading of substances that cause intoxication or make one languid, so they are haram for that reason. The Messenger of Allah (blessings and peace of Allah be upon him) said: “Every intoxicant is khamr and every intoxicant is haraam. Whoever drinks khamr in this world and dies when he is addicted to it and has not repented, will not drink it in the Hereafter.” Narrated by al-Bukhaari (5575) and Muslim (2003).
In fact, even if we assume that the one who consumes it does not reach the level of becoming intoxicated, even if he were to consume a large amount of it, at least it causes the one who consumes it to feel lethargic or languid, and anything that has such an effect cannot be permissible, because of the report narrated by Umm Salamah (may Allah be pleased with her), according to which the Messenger of Allah (blessings and peace of Allah be upon him) forbade everything that intoxicates or makes one languid.
Narrated by Abu Dawood in his Sunan (3686); Ahmad in al-Musnad (26634); al-Bayhaqi in as-Sunan al-Kubra (17399). Al-Haafiz al-‘Iraaqi said: Its isnaad is saheeh.
End quote from Fayd al-Qadeer (6/338)
Al-Khattaabi (may Allah have mercy on him) said: That which makes one languid is any drink that causes languidness and lethargy in the limbs, which is the precursor of intoxication. It is forbidden to consume it lest it be a means that leads to intoxication.
End quote from Ma‘aalim as-Sunan (4/267).
See also ‘Awn al-Ma‘bood and Haashiyat Ibn al-Qayyim (10/91) for the commentary on this hadeeth.
The questioner’s view that these drugs “have some positive outcomes” does not change the shar‘i ruling in the slightest. Even if we were to assume that this is true, khamr – which is the mother of all evils – also has some benefits. Allah, may He be glorified and exalted, says (interpretation of the meaning): “They ask you (O Muhammad) concerning alcoholic drink and gambling. Say: ‘In them is a great sin, and (some) benefits for men, but the sin of them is greater than their benefit’” [al-Baqarah 2:219]. So Allah, may He be glorified, confirms that there are some benefits in it, yet despite that the prohibition on it is emphatic, because of the negative and harmful consequences that outweigh any benefits.
Ibn Katheer (may Allah have mercy on him) said: Allah, may He be glorified, says “Say: ‘In them is a great sin, and (some) benefits for men”. As for the sin, it affects religious commitment. As for the benefits, they are worldly, because there are some physical benefits, as it aids digestion, leads to expulsion of waste, makes one more alert, and is very pleasurable, as Hassaan ibn Thaabit said before he became Muslim: “We drink it and it makes us kings and lions who do not fear meeting the enemy.”
But these benefits cannot equal its harmful and negative effects, because they impact both mind and religious commitment. Hence Allah said “but the sin of them is greater than their benefit”.
End quote from Tafseer Ibn Katheer (1/579). See also Tafseer al-Qurtubi (3/57)
The scholars of the Standing Committee stated that qat is haram, even though there may be some benefits in consuming it; that is only because its negative effects outweigh any benefits. They said: Even if we assume that there are some benefits in this qat, the definite harmful and negative consequences outweigh, and are many times greater than, any benefit there may be in it.
End quote from Fataawa al-Lajnah ad-Daa’imah 1 (22/162)
To sum up:
What is mentioned in the question about these substances not being harmful is not correct; rather they are indeed harmful, which you may find out by consulting specialists or specialist medical websites.
Moreover, they indeed lead to negative consequences, the most serious of which is intoxication or languidness, which dictates that they should be prohibited.
And Allah knows best.