And more than half of those tested at the roadside gave positive results.
About 434 roadside drug tests have led to a conviction across the force area, which includes Shropshire and Telford.
Road safety groups say they are pleased with the results and that they are able to see the new laws in practice.
New laws came in last March making it illegal to drive if a person is unfit to do so because they are on legal or illegal drugs, or because they have certain levels of illegal drugs in their blood, even if they haven't affected the ability to drive.
Police now have the power to stop drivers and make them do a field impairment test if they believe they may have taken drugs, including a roadside drug kit to screen for cannabis and cocaine.
Anyone who is unfit to drive because of taking drugs will be arrested and will have to take a blood or urine test at a police station.
Anna Higgins, of West Mercia Police, said: "Since the new legislation was introduced in March last year 838 roadside drug tests have been carried out in West Mercia. Out of these, 434 were positive or arrests."
Victoria Bristow, from the Safer Road Partnership, said: "The introduction of the new drug drive legislation a year ago and the new roadside testing kits is helping to improve the safety of our roads.
"Enforcing the new drug drive law is a year-round commitment for West Mercia Police and this remains a priority for us throughout 2016 and beyond."
One of the latest convictions for drug driving happened last week, when 20-year-old Luke Rutter was banned for driving for 17 months and ordered to pay a fine of £230, costs of £135 and a victim surcharge of £23 after being found through a blood test at the police station to have 3.9mg of cannabis in his system, almost double the legal limit of 2mg.
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