In the account given by Marco Polo in "The Adventures [or Travels] of Marco Polo" it is told that "The Old Man kept at his court such boys of twelve years old as seemed to him destined to become courageous men. When the Old Man sent them into the garden in groups of four, ten or twenty, he gave them hashish to drink. They slept for three days, then they were carried sleeping into the garden where he had them awakened.
"When these young men woke, and found themselves in the garden with all these marvelous things, they truly believed themselves to be in paradise. And these damsels were always with them in songs and great entertainments; they; received everything they asked for, so that they would never have left that garden of their own will."
When the Old Man wished to kill someone, he would take a young man and tell him they could return to Paradise if they entered his service and followed his instructions or died in his service.
From this account it is farily clear that hashish was not the substance used. First, hashish is seldom prepared in a liquid form Hassan would drug young men with a substance which "cast them into a deep sleep" from which they could not be awakened. They were then carried to a beautiful secret garden which was impenetrable and unseen by any but those intended to be his haschishin. When they awoke in the garden, surrounded by beautiful naked women and boys, they were told that they were in Paradise. After a few hours of bliss, they were again made unconscious with the unknown substance. Awakening back in the presence of "The Old Man of the Mountain" they were told that he had given them this glimpse of Paradise and that they would go to Paradise if they entered his service and followed his instructions or died in his service. Thus, he recruited an army of assassins who were the first terrorist gang.
It is from this story that the connection between the words assassin and hashish is drawn. It is said that the word assassin comes from the Arabic word haschishin for hashish user. But Hassan and his followers didn't speak Arabic; they were Persians. Assassin comes from Hassassin -- a follower of Hassan.
Hassan, in fact, was a hashish prohibitionist. He argued that the Koran's ban on alcohol was a ban on all intoxicants, so his assassins were drug free terrorists. Except in the false Paradise where they were served wine as one of the joys of heaven. So, it is desire for alcohol not hashish that helped motivate the Assassins.