A Scottish drugs advice service has questioned the decision to reclassify magic mushrooms as class A drugs.
The law clamps down on picked mushrooms and growing kits
The Drugs Act 2005 has closed a loophole which had previously only banned the prepared form of the hallucinogenic Liberty Cap fungi.
It is estimated that up to one in five 16-year-olds in Scotland have tried magic mushrooms.
But drugs service Crew 2000 said it was "nuts" to categorise them alongside heroin, crack and cocaine.
Spokesman John Arthur said: "I think we are seeing laws passed in this country through ignorance.
"There is no way you could classify the effects of magic mushrooms the same as those of heroin, cocaine, etc - I think it's nuts."
Information body Drugscope said the government did need to clarify the law on magic mushrooms but should not have made them class A.
It said that from more than 12,600 people presenting problems to drug agencies in Scotland in 2004, only 30 had used hallucinogenics in the previous month.
DRUG EXPERIMENTATION BY SCOTTISH 15-YEAR-OLDS
Magic mushrooms: 2%
SOURCE: Scottish Schools Adolescent Lifestyle and Substance Use Survey 2004
But Drugscope said local surveys suggested that young people in Scotland and Wales were more likely to have taken magic mushrooms.
Its website says: "In Scotland, for example, one in five 16-year-olds will have typically tried them."
Under Clause 21 of the Drugs Act 2005, it is now an offence to import, export, produce, supply, possess or possess with intent to supply magic mushrooms, including in the form of grow kits.
Home Office Minister Paul Goggins said: "Magic mushrooms are a powerful hallucinogen and can cause real harm, especially to vulnerable people and those with mental health problems."
The ban covers mushrooms containing the hallucinogen psilocybin but does not affect the much more rarely used magic mushroom Fly Agaric.