Saskatchewan Driver's Handbook

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Drinking and drugs combined

Many people think that driver impairment is caused exclusively by ingestion of alcohol. The truth is that the alcohol in one’s body will determine their blood alcohol level. However, if that person already has another drug in their system, the impairing effect on the functioning of the central nervous system (brain) is far greater than the impairing effect of the alcohol and the impairing effect of the other drugs combined; it is not a simple adding together of impairment, but rather a multiplier effect!

This is the synergistic effect! What it means is that the whole is greater than the sum of its parts, or 1+1 = more than two. When combining drugs and alcohol it causes a multiplying effect. This has an unpredictable effect on driving and can be deadly.

Perhaps a few real life examples would be helpful to understand this synergistic effect. Assume that a drink is:

  • 1 bottle of regular alcohol content beer; or
  • 5 ounces table wine; or
  • 3 ounces fortified wine (port or sherry); or
  • 1-1/2 ounce hard liquor (rye, rum, vodka etc.).

Combined with:

  • anti-depressants
  • gravol
  • antihistamines
  • valium

We find that there is a predictable synergistic effect related to the level of impairment caused by a certain number of drinks when combined in this manner.

Number of DrinksCombined WithEquivalent to Number of Drinks
2Antihistamine (Cold Remedy)= 4 to 5
2Marijuana (1 joint)= 5 to 6
2Tranquilizer (Valium – normal dose)= Approximately 6
2Gravol= Approximately 6

It must be noted that the dosage of the drug will have a serious effect on the “equivalence to number of drinks” estimates. Hydroponically grown marijuana, for example, is much stronger than the light dosage drug used in compiling the figures for this chart.

In all cases, the Intoxilyzer reading would indicate just the blood alcohol level caused by two drinks, but the actual impairment of central nervous system function would be equivalent to a much higher blood alcohol level.

Understanding the synergistic effect of alcohol combined with other drugs could be a lifesaver.