A Guide To Drug-Drug Interactions In Podiatry

Robert G. Smith, DPM, MSc, RPh, CPed

What About Medication Interactions With Cigarettes And Illegal Drugs?

Cigarette smoke and alcohol may interact with medications through pharmacokinetic or pharmacodynamic mechanisms. Engaging in both of these social activities can reduce the effectiveness of certain drugs or can make drug therapy unpredictable.

   The analgesic effects of hydrocodone (Vicodin, Abbott Laboratories), oxycodone (Oxycontin, Purdue Pharma) and codeine acetaminophen (Tylenol, McNeil) combination products are decreased with cigarette smoking.29 With cigarette smoking, the subcutaneous absorption of insulin is lower and an increase in warfarin clearance occurs.29 Concurrent beta-blocker use and cigarette smoking have caused pronounced decreases in heart rate and blood pressure. Finally, a decrease in sedation has occurred with zolpidem (Ambien, Sanofi Aventis) and lorazepam (Ativan, Pfizer) in cigarette smokers.29

Key Pointers On Interactions With Illegal Drugs

As podiatric physicians become more familiar with the subject of drug-drug interactions, they can avoid life-threatening events and improve patient outcome. Both pharmacokinetic and pharmacodynamic interactions of illicit drugs may occur with a range of drug types.30

   Drug interactions involving illicit drugs fall into three categories. The first category involves substances described as psychostimulants that include amphetamines, methamphetamine, ecstasy (MDMA) and cocaine.30 The second category involves cannabis.30 The last category involves illicit drugs that cannot be grouped with the other two categories.30

   All psychostimulants can increase blood pressure and may counteract the therapeutic effect of antihypertensive medications.30 Cannabis pharmacokinetic interactions occur because cannabinoids are highly protein bound. They will compete with other protein bound substances like warfarin and increase the availability of warfarin and thus increase warfarin's therapeutic effect on the body to interact with various receptors.30 The addition of methamphetamine and/or cocaine to concomitant serotonergics have the potential to cause serotonin toxicity. Central nervous system effects are additive if patients taking heroin also take a medication with central nervous system depressant properties.30

Management Strategies For Avoiding Drug-Drug Interactions

One can prevent drug interactions by avoiding concomitant administration of interacting substances or possibly employing alternative therapeutic strategies. Regularly updated reference manuals of drug interactions and computerized programs can be useful to the podiatric physician. In order to minimize drug-drug interactions involving mechanism-based CYP inhibition, it is necessary to choose safe drug combination regimens, adjust drug dosages appropriately and conduct therapeutic drug monitoring for drugs with narrow therapeutic indices.31

   One management option to control potential dangerous drug-drug interactions includes avoiding the drug-drug combination entirely, given that with some drug interaction, the risk always outweighs the benefit. It is possible to give two interacting drugs safely as long as one appropriately adjusts the dose of the object drug.

   Another drug-drug management option is to space the dosing times of each interacting drug to avoid the interaction. This option allows for the object drug to be absorbed into the circulation before the precipitant drug. Always provide information on patient risk factors that increase the chance for an adverse outcome to the patient or patient caretaker so the patient avoids the adverse effects of drug-drug interactions. Computerized drug interaction screening systems are helpful tools that the podiatric physician may use but improvement in these systems is necessary.

In Conclusion

One can predict the potential for important drug interactions based on the properties of the causative agent and the interacting agent. The majority of drug-drug interactions have known factors grounded in science. However, many healthcare providers rely solely on inductive reasoning based on personal clinical experience as a guide to the clinical importance of most drug-drug interactions.

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