A new law aimed at banning so-called legal highs could lead to people buying herbal remedies over the internet facing a seven year jail term, experts have warned the Government.
The legislation is so widely drafted that it could include substances that are "benign or even helpful", including some "evidence-based herbal remedies", Home Secretary Theresa May was told.
The blanket ban could even damage the ability of scientists to carry out research on psychoactive substances, the Advisory Council on the Misuse of Drugs (ACMD) said.
The Psychoactive Substances Bill, which is currently going through Parliament, will introduce a blanket ban on the production, distribution, sale and supply of legal highs - officially known as new psychoactive substances (NPSs) - after they were linked to scores of deaths.
More than 500 new drugs have been banned by the Government but the current system is seen as laborious as substances have to be assessed individually before they can be outlawed and manufacturers often produce new versions almost immediately after a previous form has been prohibited.
In a letter to Mrs May, the ACMD's chairman Professor Les Iversen said there were "positive aspects" to the Bill but warned of "serious unintended consequences" from the way the legislation was drawn up.
He said the legislation was only supposed to tackle new substances but "the omission of the word 'novel' has widened the scope of the Bill beyond that originally intended".
It was "almost impossible to list all possible desirable exemptions" to the legislation and "as drafted, the Bill may now include substances that are benign or even helpful to people, including evidence-based herbal remedies that are not included on the current exemption list".
Prof Iversen added that the Bill would have a "substantial impact" on the sale of many herbal medicines, with people buying them online from overseas websites possibly facing the maximum penalty in the legislation.
"Under the Bill, a large number of currently legal products would need to become registered or sellers could face legal sanction," he said.
"Purchasing a benign, possibly evidence-based herbal product from a website outside the UK would appear to attract a seven year prison tariff."
His letter also warned that the legislation could "seriously inhibit medical and scientific research on psychoactive substances" because although there is an exemption for clinical trials, it does not extend to laboratory work in academic or industrial settings.
The ACMD also had concerns that the legislation had the potential to "criminalise and apply disproportionate penalties" to many otherwise law-abiding youngsters, and the body warned that the closure of head shops could lead to the trade in psychoactive substances shifting to illegal dealers and internet sales.
The body, which provides advice to the Government on the control of dangerous or otherwise harmful drugs, warned that the legislation could face legal challenges because "the psychoactivity of a substance cannot be unequivocally proven" except through human testing.
Although laboratory tests could be carried out "such proxy measures may not stand up in court".
Prof Iversen recommended that the legislation should be changed to include a tight definition of "novel" substances rather than imposing a blanket ban.
He also called for the Bill to target commercial suppliers rather than users and called for an independent evaluation of the impacts of the legislation.
A Home Office spokeswoman said: "The Advisory Council on the Misuse of Drugs' scientific advice is greatly valued by the Government.
"The Home Secretary has received Professor Les Iversen's letter on the Psychoactive Substances Bill and welcomes the ACMD's support of a move to reduce and prevent harms and preventable deaths caused by these dangerous drugs to young people, adults, families and societies.
"We will respond to the letter and recommendations before the Bill is next debated in the House of Lords."