Max Rady College of Medicine

Concept: Concurrency

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Concept Description

Last Updated: 2005-12-05


    This concept describes how MCHP researchers have identified and defined concurrency as it relates to the prescribing of medications.

    This concept also provides a link to a document containing relevant SAS code for concurrency and other concepts related to medications. Please see the SAS code and formats section containing the following link to: Drug Variables Discussion Document - (internal access only).

    Concurrency can be measured in several ways to provide information on whether two (or more) drugs are being dispensed at the same time.

    • Drug-Drug interactions - occur when two or more drugs react with each other. Drug interactions may make a drug less effective, cause unexpected side effects, or increase the action of a particular drug. Mixing a drug for sleep (a sedative) and a drug for allergies (an antihistamine) can slow one's reactions and make driving a car or operating machinery dangerous.
    • Drug-Condition interactions - may occur when an existing medical condition makes certain drugs potentially harmful. For example, taking oral corticosteroids and/or anticoagulants increases one's risk of a serious gastrointestinal bleed, especially if either of these drugs is taken along with a traditional nonsteroidal anti-inflammatory drug (NSAID).
    • Therapeutic duplication - is the taking of two drugs that are either pharmacologically and/or therapeutically similar. Drugs for hypertension, for example, are often used synergistically to lower blood pressure; in this case, therapeutic duplication is reasonable. However, the taking of two oral contraceptives at the same time is not seen to be advantageous and could be dangerous.
    • Combinations of drugs - normally taken together. A common example is the prescribing of traditional nonsteroidal anti-inflammatory drugs (NSAIDs) in combination with gastroprotective agents (GPAs) to protect those individuals at high risk of a gastro-intestinal event (e.g., GI bleed).

Defining Concurrency

    A conservative definition requires that the 'concurrent' drug (interacting, duplicate or combination) have at least two (2) prescriptions dispensed during the span (or interval) of the primary drug. In the case of NSAIDs or statins, the NSAID or statin would be the primary drug; for NSAIDs the companion drug would be at least (2) dispensations of a GPA and for statins the interacting drug could be one of many. In other words, for the time a patient is on one drug as well as being on another drug with two or more prescriptions they are said to be concurrently taking the other drug (e.g. NSAID/GPA) or interacting with the other drug (e.g. statins and the interacting drugs).


    To calculate the span of time (duration or medication interval) that a person may be on a primary drug using statin as an example. To calculate duration of use [MED_INT] requires a combination of the total days supply (i.e., the duration between the index date and last date, or TOTDSUPP) plus the average day supply (DAYSUPP).
    • person with one prescription for statin: duration of use [MED_INT] is the days of supply [DAYSUPP] for that one prescription.
    • person on more than one prescription for statin: their last prescription dispensed in the study period is considered as the last statin date [DATEPRVD]. Two different methods are used to calculate average day supply; whichever is highest is used as the measure of duration:
      • Method 1: total daysupply [TOTDSUPP] divided by one minus total number of prescriptions
      • Method 2: last prescription's daysupply [DAYSUPP]
    index statin date June 17 1997
    last statin date November 25 1997
    last day supply 30 days
    total statin prescriptions 6
    total day supply 161 days (Nov 25 - June 17)
    method 1 duration 161 + (161/5) = 161 + 32.2 = 193 (rounded value)
    method 2 duration 161 + 30 = 191

    Since 193 is higher the finish date is Nov 25th 97 + 32 days, which is Dec 27th 97. So in this example the statin user's duration is from June 17th 1997 to December 27th 1997.

Related concepts 

Related terms 


  • Einarson TR, Metge CJ, Iskedjian M, Mukherjee J. An examination of the effect of cytochrome P450 drug interactions of hydroxymethylglutaryl-coenzyme A reductase inhibitors on health care utilization: a Canadian population-based study. Clin Ther 2002;24(12):2126-2136. [Abstract] (View)


  • drug interactions
  • pharmaceuticals
  • prescription

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