As B.C. brings in tougher penalties for drinking and driving, CTV News investigates how drivers can become impaired without taking even a sip of alcohol.

Across the province, police aren't only looking for drunk drivers -- they're also on the lookout for drivers impaired by drugs.

The number one problem, it seems, is when people start self-medicating and up their dosage of prescription drugs.

"It affects their sight. It affects their hearing. All these little things come into play when you're operating a motor vehicle," Sgt. Dave Savoy told CTV News.

Savoy is one of 100 officers in the province specifically trained to see if drivers are impaired by prescription or illegal drugs.

If police are suspicious, they look for physical signs of impairment.

"You're checking to see what your pupil size is with a card beside it," he said.

"We also take a person's blood pressure and pulse."

Walking a straight line is another test.

In the past, the problem for police was that these tests were voluntary.

Fatal crash sparks review of legislation

But drug use and driving became an issue in a crash in Aldergrove in 2002. Simon Featherston and Dayton Unger, both 16, were killed.

"Words can't describe what we're feeling. This has been devastating. It completely tore our family apart," Unger's sister Tanya said at the time.

Every year loved ones place fresh flowers at the site of the crash on the anniversary of their deaths.

The active ingredient in marijuana was found in the driver's blood, but he was acquitted on impaired driving charges.

After the trial, Crown lawyers complained that B.C. lacked legislation to control driving under the influence of drugs.

That all changed in 2008, with legislation that allows police to demand drivers be tested for possible drug use.

The law also requires requires drivers to provide bodily samples.

So far, the crackdown seems to be working. Nearly 200 people suspected of driving after taking drugs have been charged -- a huge jump from before the law came into effect.

That number's expected to soar even higher, thanks to tougher laws and increased awareness.

"I don't think that people realize the problem that drugs and impaired drivers are," Sgt. Dave Savoy said.

With a report from CTV British Columbia's Mi-Jung Lee