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Driving Under the Influence

Special Reports

By Dr Bernard Leddy Ph.D. Leddy Consultancy Ltd

Driving a motor vehicle safely requires a coordinated mixture of cognitive,visual acuity and motor reflex skills.Driving is not intuitive it is learned behaviour which can be learned well or badly.When a person drives,or attempts to drive,under the influence of psychoactive drugs and medicines they violate some aspect of the Road Traffic legislation in most jurisdictions.

In the course of my work as a Pharmacy, Pharmaceutical Science and Toxicology consultant I am often called on by legal practitioners to help them with clients who are in trouble for driving or attempting to drive under the influence of drugs, including alcohol and medicines. Roadside testing for alcohol has been commonly used for nearly 50 years and the equipment has been refined over time so that a handheld, device can now give very accurate readings of breath alcohol levels. Up to now portable roadside drug testing has been impractical. However, many new testing systems are under development and coming on the market soon, so that roadside drug testing will be as common as the ‘breathalyser’ in a few years. I believe that legal practitioners will soon have to deal with
complex and intricate drug driving cases in a way that they have not heretofore.

We need to develop new ways of professional interaction between legal practitioners and toxicologists. I am going to launch a Drug Information service for legal practitioners whichwill give access to toxicology information in real time all year round. Subscription to this service will be confined to registered legal practitioners. In the meantime you can subscribe to my free newsletter through my website www.leddyconsultancy.ie

How drugs affect driving: A guide for legal practitioners
1) Alcohol
This is a legal and freely available drug in most western countries. The permitted limit allowed for drivers is set by legislation in all jurisdictions. Here in Ireland the general limit is 50mg/100mL of blood. In the UK it is 80mg/100mL and in some Scandinavian countries it is 20mg/100mL. As driving under the influence of alcohol is an absolute offence the legal limit reached varies from place to place. Alcohol is a CNS (central nervous system) depressant.It works through two
neurotransmitters in the brain GABA and Glutamate. In the early stages of alcohol intoxication there is a reduction in inhibitions as those parts of the brain that govern restraint and self control are depressed. At blood levels up to 50mg/100mL the outward effects of alcohol may be noticed only as more talkative. Although at the upper end of that scale measurable impairment of driving skills may be emerging. The real danger area for drivers is when the blood levels of alcohol get into the 50-100mg/100mL range.At this range there is loss of motor control which will significantly affect reflexes when driving. So braking and
judging speed will be impaired.Inhibition of the senses in this range reduces self critical facilities and there may even be euphoria. In the 100-200mg/
100mL blood alcohol range both physical and mental functions become significantly impaired as the blood alcohol concentration rises through this range. This can lead to poor balance, poor judgement and poor driving performance. Many fatal accidents occur within this blood alcohol range as the driver will perform badly but is not so intoxicated as to incapable of getting into trouble. The relationship between blood alcohol level and impairment is not linear. Also, not all alcoholic
drinks contain the same percentage of alcohol.For example 500mL of an alcoholic drink of 2% alcohol will contain 10g of alcohol but 100mL of a drink containing 40% alcohol will contain 40g of alcohol so the number of pints or shorts will not in itself tell you how much alcohol was actually consumed. Alcohol is metabolised at an average rate of 18mg/hr and within the range 9-27mg/ hr. So, roughly speaking, a blood alcohol level of 100mg/100mL will only be reduced to 82mg/100mL after 1 hour. There are also enzymatic, genetic and morphological considerations which also affect alcohol metabolism. Many of the “cures’’ of folklore such as black coffee, high dose vitamin C and so on don’t speed up alcohol metabolism.It is often said that if you give strong black coffee to somebody who has had a lot to drink all you end up with is a wide awake drunk. It has been estimated in the USA that about 11% of all drivers annually have
driven over the limit.

2) Cannabis Possession and use of cannabis is illegal in most jurisdictions although it has been decriminalised in the State of Colorado and a certain tolerance to use is still present in the Netherlands. It is illegal to drive or attempt to drive after taking any form of Cannabis. The active ingredient of Cannabis is Delta 9-Tetrahydrocannabinol. The drug is obtained from the leaves and flowering parts of the herb Cannabis sativa. This plant has very characteristic diagnostic microscopical features which make it easy to distinguish from other plant drugs. It is sold as herbal Cannabis, Cannabis resin or Cannabis oil and it is usually smoked in a joint or reefer. Its use by drivers affects a wide spectrum of the skills needed for safe driving. Reflexes are slowed,also the perception of distance and speed are impaired meaning that it is much harder to respond quickly to motoring events which need avoidance or sharp braking. It also significantly
affects the drivers ability to track or stay in lane.D9THC (Delta 9-tetrahydrocannibinol) is cleared rapidly from the bloodstream and is often difficult to detect in whole blood samples after about 3 hours.It is highly lipid (fat) soluble and may persist in the system for weeks or months after taking Cannabis which is why people often fail drug screening conducted after time. A significant fact in forensic cases is that normal levels of driving performance are not regained for at least four to six hours after smoking a single joint. Following a road traffic accident the driver may have assumed that the effects of cannabis on driving skill tapers off quite quickly whereas the opposite is true. Mixing cannabis use with drinking alcohol leads, in my opinion, to a multiple of the adverse effects of both.

3) Tranquillisers Hypnotics and Anxiolytics
This is not a coherent group of drugs. However, many of them are related chemically and can have quite dramatic detrimental effects on driving skills In most jurisdictions these drugs are only available on prescription but like all psychoactive agents there would always be a black market or street trade in some of them. By far the largest group of these drugs are benzodiazepines or drugs which act like benzodiazepines. They are used in medicine as tranqullisers, hypnotics, anxiolytics, muscle relaxants and sometimes for clinical sedation and preoperative treatment. In spite of their wide clinical use they are powerful drugs. They differ widely in their physical properties, elimination and metabolism. The length of a drug’s activity is often expressed as its elimination half life. A useful
definition of half life is the time in minutes, hours or days at which the amount of unchanged drug becomes 50% of the initial plasma concentration. For example Flunitrazepam has a half life of 18-26 hours, Diazepam has a half life of 20-100 hours and Midazolam has a half life of only 3 hours. It is clear from these figures that many drivers are impaired many hours after they have taken one of these drugs. Generally the degree of impairment is directly related to the dose taken. These drugs affect driving skills by slowing reaction times, interfering with hand eye coordination and affecting judgement of distance and speed. It is worth mentioning here the socalled Z drugs, zolpidem,zopiclone and zalpelon. They are all used as hypnotics and although chemically distinct from benzodiazepines they exert their effects through the same receptor system as benzodiazepines. They start working quite quickly and have a measurable detrimental effect on driving skills often within 30 minutes. This adverse effect tapers off quite quickly and has usually disappeared by 7-9 hours after administration. These do not show up in drug screens for benzodiazepines and they can be difficult to find in forensic samples. There have been cases of drugged driving with these drugs where amnesia has been the overwhelming symptom and the subject has no memory of the journey. Impaired driving with these drugs can very often lead to proceedings under the criminal justice codes.

How Can I Help You.

I offer a full toxicology and drug information service for Lawyers.It will be fully available online later in the year but in the meantime you can contact me through my new website

Dr.Bernard Leddy 2015.

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