Drug Interactions and Its Types

Anytime you take more than one medication, or even mix it with certain foods, beverages, or over-the-counter medicines, you are at risk of a drug interaction. Most drug interactions are not serious, but because a few are, it is important to understand the possible outcome before you take your medications.

Drug-drug interactions: These are the most common type of drug interaction. The more medications you take, the greater the chance for your drug interacting with another medicine. Drug-drug interactions can decrease how well your medications work, it may increase minor or serious unexpected side effects or increase the blood level and possible toxicity of a certain drug. For example, if you take a pain medication, like Vicodin, and a sedating antihistamine, such as Benadryl, at the same time you will have an additive amount of drowsiness as both medications cause this side effect.

Drug interactions can occur in several different ways:

1. A pharmacodynamic interaction occurs when two drugs given together act at the same or similar receptor site and lead to a greater (additive or synergistic) effect or a decreased (antagonist) effect. For example, when chlorpromazine, sometimes used to help prevent nausea and vomiting, and haloperidol, an antipsychotic medication for schizophrenia, are given together there may be a greater risk for causing a serious, possibly fatal irregular heart rhythm.
2. A pharmacokinetic interaction may occur if one drug affects another drug’s absorption, distribution, metabolism, or excretion.

• Absorption: Some drugs can alter the absorption of another drug into your bloodstream. For example, calcium can bind with some medications and block absorption. The HIV treatment dolutegravir (Tivicay) should not be taken at the same time as calcium carbonate (Tums, Maalox, others), because it can lower the amount of dolutegravir absorbed into the bloodstream and reduce its effectiveness in treating HIV infection. Dolutegravir should be taken 2 hours before or 6 hours after medications that contain calcium or other minerals to help prevent this interaction. In the same manner, many drugs cannot be taken with milk or dairy products because they will bind with the calcium.
• Distribution: Protein-binding interactions can occur when two or more highly protein-bound drugs compete for a limited number of binding sites on plasma proteins. One example of an interaction is between fenofibric acid (Trilipix), used to lower cholesterol and triglycerides in the blood, and warfarin, a common blood thinner to help prevent clots. Fenofibric acid can increase the side effect of warfarin and cause you to bleed more easily.
• Metabolism: Drugs are usually eliminated from the body as either the unchanged (parent) drug or as a metabolite that has been changed in some way. Enzymes in the liver, usually the CYP450 enzymes, are often responsible for breaking down drugs for elimination from the body. However, enzyme levels may go up or down and affect how drugs are broken down. For example, using diltiazem (a blood pressure medication) with simvastatin (a medicine to lower cholesterol) may elevate the blood levels and side effects of simvastatin. Diltiazem can inhibit (block) the CYP450 3A4 enzymes needed for the breakdown (metabolism) of simvastatin. High blood levels of simvastatin can lead to serious liver and muscle side effects.
• Excretion: Some non-steroidal anti-inflammatory drugs (NSAIDs), like indomethacin, may lower kidney function and affect the excretion of lithium, a drug used for bipolar disorder. You may need a dose adjustment or more frequent monitoring by your doctor to safely use both medications together.

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