Jose Antonio Guerra Guirao, Complutense University of Madrid and Francisco López-Muñoz, Camilo José Cela University (The Conversation)
Drug interactions are a real health problem that we pay more and more attention to. It is explained, on the one hand, by the demographic trend of our society towards a progressive aging of the population. Although this increase in life expectancy goes hand in hand with an improvement in the quality of life, it also suggests that more and more drugs will be used simultaneously for long periods of time.
On the other hand, scientific advances mean that the therapeutic arsenal is increasing significantly for the treatment of different pathologies. Furthermore, it is becoming more common for several drugs to be prescribed for the same disease.
What are drug interactions?
An interaction is said to exist when a medicinal product does not exert the expected effect when administered simultaneously, or successively, with another medicinal product, medicinal herb, food, beverage, or environmental contaminant. The problem may arise due to excess, default or due to a different reaction than expected.
However, there are certain physiological conditions, such as pregnancy, or pathological conditions, such as kidney or liver failure, which can also condition the appearance of drug interactions. Mainly because they affect the behavior and metabolism of drugs in our body.
It should be borne in mind that the number of drugs administered together directly influences the incidence of interactions. We can safely affirm that when two drugs are co-administered it is possible for a drug interaction to occur, when three drugs are administered simultaneously it is highly probable, and when four or more are administered at the same time, the rare thing is that there is no interaction.
Furthermore, we must not forget or underestimate potential and actual interactions with products considered “safe” and “harmless”. We are referring to medicinal plants, food, vitamins and minerals, or other substances, including tobacco or alcohol. In fact, the interactions between certain drugs and grapefruit or grape juices are becoming better known, and also more frequent than previously imagined.
A serious problem
In industrialized countries, adverse drug reactions rank between the fourth and sixth leading cause of death. Recent studies reveal that up to a third of these adverse reactions may be due to drug interactions.
The numbers speak for themselves. In the United States, for example, drug interactions account for 3% of all hospitalizations annually), generating an expense of $ 1 billion. As for Spain, they represent 5-15% of hospitalizations in the elderly population and more than 45% of hospital readmissions.
Most commonly, they have negative consequences. Either because they increase the action of one or more of the drugs taken by the patient, causing adverse reactions or side effects, or because they reduce the effect of one or more of the drugs consumed, causing the treatment to fail.
At other times, drug interactions can have a positive effect and even be sought “therapeutically.” An example of this occurs in drug-resistant depressions, when the physician associates two different types of antidepressant drugs that can enhance their action.
Pharmacokinetic interactions
These types of interactions are produced by alterations or modifications of the processes that drugs undergo within our body. For example, the medications we routinely take for burning, which work by reducing or neutralizing gastric acids, modify the pH of the stomach. In this situation, the absorption of other drugs, such as hypnotics or antiepileptics, can be altered.
It also happens that many drugs are destroyed and inactivated (metabolized) by certain enzymatic substances mainly present in the liver. The activity of these liver enzymes can be modified by the action of some medicines, increasing or reducing their activity. As a consequence, it may happen that another drug taken simultaneously is inactivated in a faster way, which would reduce its effect considerably. Or also that it is inactivated more slowly than normal, which would increase the accumulation of the drug in the body, increasing the risk of toxicity.
Among the drugs that cause an increase in the activity of these enzymes, some antiepileptics stand out, and among which many antidepressants decrease it, in addition to drugs used in fungal infections.
But not only medications are involved in this process. The chemicals in cigarettes can increase the activity of some liver enzymes. This implies that tobacco consumption decreases the effectiveness of some drugs. Likewise, some foods may also be involved, such as grapefruit, which can decrease the speed with which the body breaks down certain medications, such as antiarrhythmics, anxiolytics or the well-known statins, used to reduce cholesterol levels.
Lastly, we know that certain medications can modify the rate of renal elimination of others. For example, high doses of vitamin C increase the acidity of the urine, and this can modify the elimination ratio and the activity of quinine or amphetamines.
Among the most dangerous interactions of this type are those involving acenocoumarol, an anticoagulant medication commercially known as Sintrom, taken by many patients who have suffered a cardiovascular event. Its plasma concentrations are easily affected with the simultaneous taking of various drugs.
Pharmacodynamic interactions
These types of interactions take place when different drugs are administered that act on the same place.
The potentiation of the effect can take place when, inadvertently, two drugs are taken that, acting in different places, cause the same effect. For example, when we consume two antihistamine agents for the treatment of allergic symptoms with sleeping pills, or any of them with alcohol). In these cases, an increase in the depressant effects of the nervous system takes place, with drowsiness and sedation.
Another possibility is that, when administering medicines with opposite actions, their effect is reduced or canceled. This is the case of ibuprofen, a widely used anti-inflammatory, which can reduce the effectiveness of diuretic drugs used in the treatment of high blood pressure.
Two plus two do not add up to four in pharmacology
Medicines routinely contribute to improving our quality of life. Not only because they alleviate pain, but also because they help fight infections, control symptoms of chronic conditions such as high blood pressure or diabetes, and even reduce the risk of death from serious diseases, including cancer.
However, we must not lose sight of the problems that drug interactions can cause, especially in elderly patients, which can condition the loss of efficacy of a treatment. Or, what is worse, the appearance of serious adverse reactions. And, in this field, two plus two almost never add up to four.
Jose Antonio Guerra Guirao, Professor of Pharmacology and Toxicology. Pharmacy faculty. Complutense University of Madrid., Complutense University of Madrid and Francisco López-Muñoz, Professor of Pharmacology and Vice-Rector for Research and Science, Camilo José Cela University
This article was originally published in The Conversation. Read the original.
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