Habitual use of alcohol or tolerance plays a role in how alcohol affects a particular person.
Tolerance is characterized by two major factors:
Tolerance is a result of modifications in the nervous system and metabolic changes. The two major mechanisms responsible for the development of tolerance are (1) a decreased amount of alcohol reaching the area when the effect is produced and (2) the lowered responsiveness of the tissue to the alcohol.
There are several different types of tolerance.
Acquired tolerance is the result of alterations in the way the body absorbs, distributes, and eliminates alcohol (dispositional tolerance), and resistance of the cells of the body to the effects of alcohol (cellular tolerance).
Acute tolerance occurs when the individual is less impaired at the same blood alcohol concentration (BAC) during the falling or elimination portion of the alcohol curve than during rising or absorption portion.
Behavioral tolerance occurs when the individual learns to mask the outward effects of intoxication by repeatedly practicing physical skills at elevated alcohol levels.
Several hypotheses have been proposed to explain alcohol tolerance.
One hypothesis involves the mechanism of alcohol action on the cellular membranes. Alcohol interferes with the sodium and potassium channels required for the transmission of nerve cell impulses. Alcohol consumption over long periods of time is said to increase the activity of the enzyme responsible for activating the sodium and potassium channels, and thus helps compensate for the alcohol interference.
Another hypothesis states that repeated alcohol exposure causes the cell membranes to become more rigid by increasing the cell’s content of saturated fat or cell membrane’s cholesterol content.
Yet another hypothesis states prolonged alcohol consumption changes the neurotransmitters, GABA and serotonin, and brain alcohol dehydrogenase activity.
Regardless of the mechanism behind tolerance, one thing is clear: drinkers develop desensitization to the effects of alcohol such that greater doses are required to achieve the same effect as a prior time. In fact, heavy drinkers may experience greater stimulant-like and fewer sedative-like and aversive subjective effects after drinking than novice drinkers.
Alcoholics develop an increased tolerance to alcohol at BACs that are extremely high, including levels generally considered to be potentially fatal. One study found that out of 54 subjects with BACs of 0.20% or higher, 24% showed no signs of clinical intoxication. [Perper JA, Twerski A, Wienand JW, Tolerance at High Blood Alcohol Concentrations: A Study of 110 Cases and Review of the Literature, Forensic Sci 1986 Jan; 31(1):212-221, p. 213.]
In another study, out of 32 patients evaluated, seventeen (53%) had levels that exceeded 0.30% but exhibited little clinical evidence of intoxication. [Davis AR, Lipson AH, Central Nervous System Tolerance to High Blood Alcohol Levels, Med J Aust 1986 Jan 6;144(1):9-12.]
Even if some signs and symptoms of impairment are noted, there is no real correlation between BAC and performance for individuals with tolerance.
Tolerant and/or frequent drinkers, at blood alcohol concentrations between 0.08% and 0.10%, have a lower probability of accidents than individuals at the same levels who are infrequent drinkers. One study found that between two drivers with the same alcohol level, the one with the lowest daily consumption of alcohol was much more likely to be involved in a collision. [Borkenstein, R, Crowther, R, Shumate, R, Ziel, W, Zylman, R, The Role of the Drinking Driver in Traffic Accidents, 1964, Allen Dale (ed.) Department of Police Administration, Indiana University.]
Acute tolerance (called the Mellanby effect) occurs when a person appears to be less under the influence at a specific BAC when the person’s BAC is falling after reaching its peak, than when it is rising before it peaks. In other words, the person is less impaired longer after drinking than sooner after drinking even though the person’s BAC is the same at both times.
A study evaluated acute behavioral tolerance by dosing subjects up to 0.10% BAC, and then dosing a second time to 0.10% when the BAC from the initial dose had deceased to 0.05%. Acute tolerance for some behavioral tests was demonstrated by an improved performance after the second dose as compared to the first dose when BACs were the same. [Wilson JR, Erwin VG, McClearn GE, Plomin R, Johnson RC, Ahern FM, Cole RE. Effects of ethanol: II. Behavioral Sensitivity and Acute Behavioral Tolerance, Alcohol Clin Exp Res 1984 Jul-Aug;8(4):366-74.]
Another study compared the behavioral performance on the rising side of the curve with the falling side of the curve for reaction time and visual skills. The authors found that reaction time and depth perception showed more impairment in the rising BAC than in the falling BAC. [Nicholson ME, Wang M, Airhihenbuwa CO, Mahoney BS, Christina R, Maney DW, Variability in Behavioral Impairment Involved in the Rising and Falling BAC Curve, J Stud Alcohol 1992 Jul; 53(4): 349-56.]