The force has promised to deal with the use even of Class A drugs, such as heroin and cocaine, in a "supportive, not punitive" manner. Only officers caught in possession of illegal drugs will be dismissed.
Some employers, including London Underground, immediately dismiss employees who test positive for drugs.
A spokesman for Merseyside Police said the aim of the testing was to help officers come forward for counselling and rehabilitation.
Many other forces, including the Metropolitan Police, are expected to follow suit shortly.
The policy has been backed by the rank and file, represented by the Police Federation. "We are against all forms of drug taking and we have no serious concerns about testing officers," said a Federation spokesman.
Next month, the police's leading authority on drug testing, Andrew Timpson, Chief Constable of Warwickshire, will present recommendations to the Association of Chief Police Officers. "Drug testing is growing in industry generally and an increasing number of forces are looking at it as part of their welfare policies. We also cannot ignore the fact that for the younger generation drug use is a lifestyle thing," he said.
There is concern in police forces not only at the use of drugs such as cannabis, Ecstasy and cocaine, but also at the misuse of muscle-building steroids. The legal position is complicated because possession of steroids is not proscribed.
Merseyside Police were at the centre of a drugs scandal last year, when two officers were suspended in a crackdown on muscle-building drugs. Local drugs workers claimed that other officers were implicated but that the issue was covered up.
Merseyside Police refused to say whether there was a connection between the incident and the introduction of the new drug-testing policy, though its screenings will also test for steroids. Traditionally, officers with drug problems leave the force quietly. However, at least one of the officers suspended in the Merseyside steroid investigation is still at work.
The steroid phenomenon is driven by officers who feel they can enhance their physical presence. But steroid abusers risk numerous health problems, including kidney and liver disorders, heart disease and exposure to HIV through sharing needles. Coming off the drugs can cause acute depression and even suicidal thoughts.
Jobs on the line, page 6