Although blood alcohol levels are an important measure in understanding the effects of alcohol, there are very important differences depending on how long the blood alcohol level is elevated. There is an acute tolerance to alcohol effects, most clearly observed in measures of motor performance. The same blood alcohol level early in exposure to alcohol produces more motor impairment than later in exposure, even though the blood alcohol levels are the same.

Acute Tolerance to Alcohol.

Following a single dose of alcohol standing unsteadiness decreased as blood alcohol increased but it returned to normal while blood alcohol levels were still elevated. 40 subjects received 0.63 - 1.42 gm of alcohol per kg in water (about 3-6 drinks) on an empty stomach. Standing unsteadiness was measured with one foot in front of the other (modified Romberg test) with the use of a tracing of movement. (Constructed from data in Goldberg L, et al, Acta Physiologica Scandanavica Supplementum, 5: 1-128, 1943)
Early after alcohol ingestion (35 min) both reaction time and accuracy are impaired. However, about 95 min after drinking alcohol only accuracy is impaired even though alcohol blood levels are the same. This is evidence that different aspects of behavioral function adapt at different rates following elevated blood alcohol levels. The effects of elevated blood alcohol levels on higher cognitive function (accuracy and learning) outlast the effects on motor function (reaction time).

Reaction Time and Accuracy of Performance on Declarative Memory is Impaired by Alcohol Differently on the Rising and Falling BAC.

Reaction time is impaired on the rising BAC but not the falling BAC while accuracy is impaired on both the rising and falling BAC. 2 groups of 10 male social drinkers (mean age 21) took a battery of neuropsychological tests before and after drinking alcohol 0.65 h/kg or Placebo. The 25 minute testing for the rising BAC started at 35 min after drinking the alcohol, (BAC 0.081 to 0.093 gm/100ml) and the falling testing started at 95 min after drinking (BAC 0.086 to 0.079 gm/100ml). Of the 4 measures of reaction time across the 14 tests, 3 were impaired during the rising BAC and 0 were impaired during the falling BAC. Of the 10 measures of accuracy across the 14 tests, 2 were impaired during the rising BAC and 5 were impaired during the falling BAC, indicating that reaction time is sensitive to rising BAC and accuracy is sensitive to falling BAC. (Constructed from data in Schwelzer TA, et al, Neuropsychopharmacology, 31: 130, 2006)