Like any energy in nature, consciousness is an energy that can be conditioned or filtered. Pure, unfiltered consciousness is therefore capable of perceiving the highest, most pure regions of nature, whereas conditioned consciousness only perceives those regions that correspond to the conditioning.
Since at our level our consciousness is heavily conditioned—by our attachment to physicality and by our entrenched psychological habits—we only perceive through our conditioning: that is, we only see physicality and what our psychological habits allow.
If we learn to free our consciousness of those conditions, we can see more, relative to the degree of freedom we access. This is why we learn how to meditate, and to free the consciousness from our psychological conditions.
Many people, guided by their psychological habits, want to access vision of more than physicality, and rely on chemicals such as those you mentioned (whether those chemicals are made in a lab or occur naturally in a plant). Visions are possible from such influences, but do nothing to altar the fundamental problem: the consciousness remains conditioned, and even becomes more conditioned: addicted to the substance or the experience the substance gives. Thus, ones problems are not resolved, they are worsened, Moreover, the visions received are still filtered by the psychological conditioning, so that what is seen is not objective (true) but subjective (influenced by the psychological conditioning of pride, lust, envy, fear, etc). This is why drug users (and plant users) increase their dependence on external elements, and increase their pride, lust, fear, envy, etc. Visions and experiences acquired through drugs are perceptions of the levels that correspond to our conditioning: since our minds are corrupted by many defects, drug users see via the levels of those defects; i.e hell. Drug-induced visions are visions of the hell realms. A drug user can never see the heavenly realms. They may think they see it, but are only seeing projections of their desires. Conditioned consciousness cannot see the unconditioned realms.
There are certain rare occasions where a MASTER may treat a student using the medicine of certain plants. Such treatments are generally conducted in the internal worlds, not physically.
Therefore, we reject the use of any and all drugs, whether made in a lab or in a plant. We know that the consciousness has the power to access genuine visions and conscious knowledge that is objective, pure, and truthful, and can lead the soul to freedom from suffering. Drugs cannot.
Meditation leads to experiences that are millions of times more powerful than any drug. Meditation awakens and develops the full power of the human consciousness, and unites it with the Divine. A person who follows this development becomes a master, a god. A drug user remains a drug user, a slave of nature, and weak.
"Obviously, drugs annihilate alpha waves. Then, unquestionably, the intrinsic connection between mind and brain is lost. This, in fact, results in total failure.
A drug addict turns vice into religion. Being misled, he thinks he experiences what is real under the influence of drugs. Unaware that the extra-perceptions produced by marijuana, L.S.D., morphine, hallucinogenic mushrooms, cocaine, heroin, hashish, tranquilizers in excess, amphetamine, barbiturates, etc., etc., are merely hallucinations..." - Samael Aun Weor