The six-page letter, written to appear as a leaked memo, is signed by the 'Joint Strategy Committee Chair', and was addressed to 'UK Cartel CEOs only'.
The document makes a series of bizarre claims about several charities including Drugscope and Alcohol Concern. The hoax letter implies that Drugscope is a key partner for a cartel of pharmaceutical companies that stand to make huge profits from the legalisation of hard drugs.
The letter also states that the corporate cartel aims to take control of the supply, production, marketing and sale of all drugs; to ensure continued expansion of drug use for medical, behavioural control and recreational purposes; and to maximise the financial contribution of the Government towards the strategy.
It also implies that Drugscope's education work encourages people to experiment with drugs.
Assembly members received the letter over the Easter weekend.
Martin Barnes, chief executive of Drugscope, described the letter as "a crude, silly, but nasty, attempt to besmirch the name of Drugscope and other reputable organisations", and said he would be taking legal advice over the incident.
"It's obviously a hoax, but it makes some fairly libellous claims," said a spokeswoman for the charity.
"Anyone who saw it would know instantly that the claims are laughable. Drugscope does good work, and it is particularly annoying to see our education and prevention work being twisted into claims that we are preparing the drug users of the future.
"The indication so far is that no-one who has seen the letter takes it seriously, but the chief executive will decide next week whether this is a police matter. In any case, he will be contacting Assembly members to reassure them about the nature of our work."
A spokesman for Alcohol Concern said he was unaware of the letter, but would not want to give any validation to it.