Some say that the Devas prevented the Asuras from obtaining the Amrita made by the "churning of the ocean of milk". However, your observation is correct in that the Asuras obviously were able to obtain Them.
This assumes that the substance in question was marijuana.
You mentioned other substances too, and from what little I know - the proto Indo-Iranians also said the same about the Soma (pron. Sauma).
It is worthy to note that the substances are mentioned in two cultures - Indo-Iranian and North Indian.
I do not think this substance existed outside of these areas and so any research into territories north of the Himalaya's requires some explanation.
Having said this, there is a well known tradition among Hinduism regarding the "fair headed races of the North". No doubt, these fables were embellishment from the Indo-Aryan peoples (fair headed people who migrated to India through the Indus valley and settled North India in ancient times). If we take this into consideration then there is a possibility that the substance in question is actually something from another land - but this remains a slim opening. The reason being is that India is well known for its Ayurveda and it's understanding of the human body in terms of medicine and good health/wellbeing. In fact no other culture from ancient times had expended such effort into understanding the human body, it's ailments and treatments and India spawned the first culture which actually championed such a cause - quite an achievement as Ayurveda still survives and is practiced today (I, myself take Mukta Vati tablets from the Divya Pharmacy when I feel restless).
There are many such plants in India (a naturally rich and fertile land) which could be Soma. One of which is Aswaghanda, a digestive aid said to increase vitality and strength if taken regularly - this is very important to know since in Indian Ayurveda the belief is that all illnesses of the body stem from the stomach. If the stomach is in good health then the body will follow.
With this understanding we can start to understand the medical logic which Ayurveda follows.
In some Vedic texts, the Soma is known to be a grass type seed - I present Fennel seeds, also a digestive aid... problem with this logic is that there are many such herbs, plants and seeds which are known to aid digestion and strengthen the stomach.
I theorize that the idea of Soma being an unique plant was no longer a consideration once Ayurveda was widely established in India - many such plants, seeds and herbs were now competing with each other and the Soma mentioned in ancient text and so - with the perfection of Ayurveda (blending of plant, seed and herb), such ideas as Soma being a miracle plant eventually faded into obscurity as the science of Ayurveda became more practiced and the results of which, more widely applauded.
Today, the reputation of Ayurveda being a medical alternative has reached far and wide and no corner of the globe is without it. Soma, has left a legacy in Ayurveda which has perfected the traditions of Ayurveda in India.
I hope this helps.
Scimi
EDIT: as an afterthought it did occur to me that Hindu's do not eat meat and have the highest stat of Anemia in the world due to their being very little to no iron in the Hindu diet.
Hence I think another opening for you would be to investigate which plants, seeds and herbs contain iron or other ferrous elements which when ingested - would empower the person with what would seem to them to be "super human strength, and vitality".
Bless,
Scimi