There is a tolerance period linked with the use of most hallucinogens, and if you have taken some, it can be from 5 to seven days before you can feel it's full effects again.
Also, the use of some medication can block the effects of hallucinogenic drugs.
And possibly some people are just immune.
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Nicolas Langlitz has written: 'Neuropsychedelia' -- subject(s): Neuropsychopharmacology, Hallucinogenic drugs and religious experience, Hallucinogenic drugs, Research
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Yes.
When you touched hallucinogenic drugs...
It is theoretically possible, however, unlikely, much the same way that it is unlikely for anyone to be immune to other drugs and toxins. Some people will have higher resistances and tolerances, though.
Altered states typically refer to use of psychoactive or hallucinogenic drugs.
It would be very valuable in the right mindset.
Quite likely. The ancient Egyptians used hallucinogenic plants in religious rituals.
Yes it is indeed a hallucinogenic drug more to be found at the Related Link.