Tony Blair has pledged to crack down on those who "peddle the misery of drugs" as he launched a set of new measures.
Tony Blair takes a drugs test at a police station in Slough
Drugs were "every parent's nightmare", the prime minister said after unveiling plans which give tougher sentences for those who deal near schools.
Those arrested for some offences would face a compulsory drugs test to get more addicts into treatment, he said.
The Tories said the plans did not go far enough, while the Liberal Democrats described them as "constructive".
Mr Blair said he would "bear down" on street dealers and organised crime as he launched the policies alongside his home secretary at a police station in Slough, Berkshire.
He said: "Every parent's nightmare is that their child gets mixed up in drugs.
"You can tell them about it and try and be as tough as you can with them and say how dangerous it is but - especially with teenagers - you are not always seeing what they are doing, what they are up to, so of course you really worry about them."
In front of the cameras Mr Blair underwent a mouth-swab drugs test for cocaine and heroin. The result was negative.
"If you are a drug addict engaged in crime, you will be offered a way out through treatment and help," he later told a news conference in the station canteen.
"If you refuse that offer, it will be made more difficult for you at every stage in the criminal justice system," he said.
Tony Blair in a cell in Slough police station where the plans were launched
One of the government's proposals is to make it an aggravating factor on sentencing for a dealer to sell drugs near schools or use children as couriers.
Ministers also intend to stop dealers claiming that the drugs they are caught with are for their own use.
There will be a presumption where people are found with larger quantities of drugs that they intend to supply - a far more serious offence than possession.
A new target will be set to increase the number of drug offenders entering treatment in the worst-affected areas, from 1,500 per month to 1,000 each week by 2008.
The bill is expected to introduce powers to test offenders for crimes such as theft on arrest rather than when they are charged.
People with a positive test would then be forced to undergo an assessment by a drug worker.
Tory leader Michael Howard said he wanted a 10-fold increase in the number of residential drug rehabilitation places to proved "young, hard drug users" to 50,000 places each year.
For the Lib Dems, Mark Oaten said that the government's proposals were "constructive" but warned they might "fall by the wayside" before the next election.
"The government rightly talks about the need to expand treatment, yet their own figures for numbers in treatment are misleading," he said.
Mr Howard said the Conservatives would provide the resources in every local education area for drug testing equipment.
"In 1997 Tony Blair said 'I am determined to tackle the menace of drugs and the devastating effects they have on our communities,'" he said.
"He was right. But we've had a lot of talk. Reality has not matched his rhetoric. Tony Blair has let people down.
Conservatives will take action to get youngsters off drugs."
The Conservatives would restore cannabis as a Class B drug if elected, the party said.
The head of charity DrugScope said expanding treatment services was not enough - the drop-out rate for people undergoing compulsory treatment had to be tackled.