The Science of Alcohol Metabolism and Tolerance

Sometimes, as part of a zealous attempt to obtain a conviction in a drunk driving case, a prosecutor may hint at, attempt to elicit testimony about, or perhaps even simply argue that the defendant is a drunk.  While a prosecutor will rarely actually say this, they will often suggest, intimate or actually assert that the defendant is tolerant to alcohol.  But these statements are based on a lack of real knowledge about alcohol metabolism and tolerance.

The intoxication of an individual depends on the absorption, distribution and elimination of alcohol.  Broadly speaking, this movement of alcohol through the body, from the moment of consumption to the moment of elimination, is referred to as the “pharmacokinetics” of alcohol.

The rate of absorption, distribution and elimination varies greatly between individuals and can have a substantial effect on an individual’s level of intoxication and chemical test results.  Despite the substantial variability of these factors, the chemical tests used to determine the intoxication of an individual are based on the ‘average’ person under constant conditions.  The ‘average’ person is created using a number of assumptions that do not take into account the substantial individual variation.[i]

The impact of alcohol on the central nervous system (CNS) is broadly referred to as the “pharmacodynamics” of alcohol.  In this regard, “[T]he intensity of the CNS effects of alcohol is proportional to the concentration of the alcohol in the blood.”[ii] Thus, at blood concentrations of .01 – .05 the state of alcohol influence is considered “subclinical”, and the signs and symptoms are not apparent or obvious.  At concentrations of .03 – .12 the state of alcohol influence is “euphoria,” with such signs and symptoms such as some sensory-motor impairment and slowed information processing…  Once an individual obtains a bodily alcohol concentration of .18 – .30 we begin to see increased muscular incoordination, staggering gait and slurred speech. [iii]

Drug tolerance may be defined as a diminution of effectiveness after a period of continuous or large-dose administration of the drug.  Tolerance may result from two separate mechanisms: dispositional (metabolic) or functional.  In metabolic tolerance the drug is metabolized or inactivated at an increased rate after chronic administration.  Thus, a given dose produces lower blood levels after tolerance has developed.  With functional tolerance, an actual change in the sensitivity of an organ or system to the drug occurs so that with repeated administration, higher doses and higher blood levels are required to elicit the same effect.[iv]

There is little debate as to whether or not humans develop metabolic tolerance to alcohol and this is best represented by individual variations in the rate of alcohol elimination which span from .009 – .036 g/mL/h.[v] For the vast majority of individuals the rate of elimination spans from .01 – .025 g/mL/h.  After the development of metabolic tolerance, such as that found in alcoholics, the rate of elimination after a drinking binge is likely to exceed .025.[vi]

What is much less clear scientifically, is if, and to what degree, humans develop functional or behavioral tolerance to alcohol.  Scientific studies have shown that the repeated performance of a particular task in association with alcohol consumption can lead to the development of a form of adaptation referred to as “learned” or “behavioral” tolerance.[vii] Learned tolerance can reduce the alcohol-induced impairment that would ordinarily accompany the performance of that particular task.[viii] However, when conditions change or when something unexpected occurs, the tolerance acquired for that task can be negated.[ix]

These findings may be applicable to the performance of tasks involved in drinking and driving.[x] A driver who has developed behavioral tolerance to driving a familiar car over a particular route under routine circumstances may drive without being involved in a crash, despite consumption of some alcohol.[xi] However, when encountering a novel environment (e.g., a detour) or an unexpected situation (e.g., such as a bicycle darting in front of the car) this same driver would be at the same risk for a crash as a novice driver at the same BAC due to lack of prior learning opportunities for these unexpected events.

Further posts will explore the legal reasons why tolerance evidence should necessarily be precluded in a drunk driving trial.

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[i] Nichols, Drinking/Driving Litigation, § 14:1, West Group (2004).

[ii] Jones, Garriott’s Medicolegal Aspects of Alcohol, Lawyers & Judge’s Publishing, p. 29, (2009).

[iii] Id. at 28.

[iv] Id. at 39.

[v] Id. at 92.

[vi] Id. at 93.

[vii] Vogel-Sprott, M. Alcohol Tolerance and Social Drinking: Learning the Consequences. New York: Guilford Press, 1992.

[viii] Id.

[ix] Glencross, D.; Hansen, J.; & Piek, J. “The effects of alcohol on preparation for expected and unexpected events.” Drug and Alcohol Review 14(2):171-177, 1995.

[x] Vogel-Sprott, M. Alcohol Tolerance and Social Drinking: Learning the Consequences. New York: Guilford Press, 1992. Also see, Sdao-Jarvie, K., & Vogel-Sprott, M. Response expectancies affect the acquisition and display of behavioral tolerance to alcohol. Alcohol 8(6):491-498, 1991.

[xi] Id.

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