The risk of being responsible for a fatal road accident increased as the blood concentration of cannabis increased, with double or triple the risk depending on the amount of the drug used, a recent study found.

The research, published by the British Medical Journal today, comes after the Home Office published proposals suggesting that people caught by police with up to 500 cannabis joints on them are likely to escape drug dealing charges.

A study published in the British Journal of Psychiatry has also shown that almost half of all patients who were treated for cannabis-induced psychosis went on to develop some form of schizophrenia.

Blood Concentration

The latest research, by the French National Institute for Transport and Safety Research, looked at 10,478 drivers who were involved in fatal crashes between October 2001 and September 2003.

All the drivers had compulsory tests for drugs and alcohol.

The researchers found that 681 drivers tested positive for cannabis (7%) and 2,096 were found to have alcohol in their blood (21.4%). In total, there were 285 drivers who tested positive for both substances (2.9%).

The researchers said, "Driving under the influence of cannabis increases the risk of involvement in a crash. However, in France, its share in fatal crashes is significantly lower than that associated with positive blood alcohol concentration."

They said that the risk of being responsible for a fatal crash increased as the blood concentration of cannabis increased, with double or triple the risk depending on the amount of the drug used.

Evidence of a Causal Effect

The researchers also found that more men were involved in crashes than women, and they were also more likely to test positive for both cannabis and alcohol -- as were younger drivers and those riding mopeds or motorcycles.

They said that the results showed that the influence of cannabis was significantly linked to the dose, and showed evidence of a causal effect between use of the drug and crashes.