The number of adults seeking medical help for cannabis addiction has risen by 50% since Labour downgraded the drug, health authority figures show.
Cannabis is the most commonly used illegal drug
Over 16,500 adults sought treatment for cannabis use in England in 2006/7 compared with 11,057 two years earlier when the drug moved from Class B to C.
Currently 500 adults and children are being treated in England each week.
The Department of Health said the rise in treatment reflected improvements in drug treatment and not cannabis use.
But Marjorie Wallace of the mental health charity SANE said the reality was now that hospital beds were filling up with people suffering the effects of cannabis addiction, depriving others from treatment.
Figures also show hospitals are treating more than 1,000 cases of alcohol-related problems a day.
Drug and alcohol campaigners said the data showed young people were particularly at risk.
The number of children needing medical help in hospitals or clinics after smoking cannabis has risen to more than 9,200.
And among under-18s there was a rise of 40% in those seen by doctors in casualty or in the consulting room for alcohol-related problems over the last seven years.
The government is currently considering whether to reverse its reclassification.
Advocates of downgrading cannabis say the risks are low compared with alcohol or tobacco.
But others say the drug should be moved back to Class B, in line with drugs like amphetamines. This would mean people in possession could be jailed for five years and charged an unlimited fine.
For possession of Class C drugs, such as steroids, the maximum penalty is a two-year prison sentence, but charges are rarely brought against people found with small quantities of such drugs.
The British Medical Association said the figures strengthened its opposition to the drug.
A spokesman said: "This is a drug that is mostly smoked, so that can cause lung damage and cancer.
"There are also concerns about the potential negative effect cannabis has on users' psychiatric state."
A Department of Health spokeswoman said: "The increasing numbers accessing treatment as a result of cannabis use demonstrates that treatment services are getting better at engaging those in need of treatment, despite an overall reduction in the prevalence of cannabis use."
Addaction, a charity that treats people with drug problems, warned that young people often use cannabis at crucial development stages in their lives. "It does have serious impacts on mental health and physical development," said a spokesman.
Marjorie Wallace, of the mental health charity SANE, said the figures were shocking, but not surprising.
She said: "The reality is now that hospital beds are filling up with people suffering the effects of cannabis addiction, depriving others from treatment.
"SANE been saying for years that cannabis is a dangerous drug; for some young people regular use can double the risk of developing later schizophrenia.
"You only need to see one person whose mind has been distorted and life irreparably damaged, or talk to their family, to realise that the headlines are not scaremongering but reflect daily, and preventable, tragedies."
Alcohol Concern said the figures on alchol abuse provided ample evidence for the need for a very serious look at the price and affordability.
Spokesman Don Shenker added: "The number of off licences still selling alcohol to under age drinkers shows that a much stiffer penalty regime is required."