IN THE past two years, drug-driving rates have overtaken that of drink drivers.

In Warwick, it’s estimated 30 to 40 people driving under the influence are charged every fortnight, and less than 10% of those are drink drivers.

The rest, test positive for drugs.

Unlike alcohol, which is out of the system in about 12 hours, drugs stick around longer, in some cases, for weeks.

Professor of Biomedical Sciences Lindsay Brown of USQ and Professor of Pharmacy Greg Kyle of QUT explained why.

“The short answer for how long drugs stay in the system is, it depends,” Prof Kyle said.

“Drugs are metabolised by the liver, and kicked out by the kidneys.

“They leave the system in a measurement of half lives, and as a general rule of thumb it takes five half lives for a drug be inactive.”

Prof Kyle and Prof Brown said marijuana had a half life of about 40 hours.

“From an effect perspective, that means it can take 10 days or more to leave the system,” Prof Kyle said.

“However, if we are talking about how long something is detectable, the more that is taken, the longer it takes to leave.”

That means blood tests can still prove positive for marijuana after a fortnight.

Prof Kyle said the half life of methamphetamines was between eight and 20 hours.

“It can depend on a number of things, such as purity and and specific chemicals,” he said.

“You start with methempahtamine and the body converts it to amphetamine.”

That means meth takes up to five days to leave the system, though it is also dependent on dosage.

Prof Brown appears in court as a an expert drug witness, and said that drug tests could not provide any more than proof a person took a drug at a certain time.

“The test shows that the person did take drugs at some stage in the past,” he said.

“Even though it’s inactive in the body, and has no effect on mood, it can show up on tests.

“The test doesn’t provide evidence as to when the drug was taken.

“The problem is how this information is interpreted.

“You can’t say, because that person took the drug, they drove erratically.”

Prof Kyle agreed, saying the tests only detected chemical evidence the drug had been active in the system.

“From a therapeutic perspective, days later the drug might not be in effect, and is effectively out of the system,” he said.

“However, from a chemistry perspective, it’s still there.”

In Warwick, police conduct random drug testing via a saliva sample to test for the presence of THC (the active ingredient in marijuana), methylamphetamine (speed or ice) and MDMA (the active ingredient in ecstasy).

There is zero tolerance for driving under the influence of illegal drugs carrying a penalty of up to $1649, licence disqualification for up to nine months or a term of imprisonment for three months.