Authors : Leonor Rodrigues and Joana Simões (pharmacists ) A drug interaction occurs when the effect of a medicine is changed by the presence of another substance administered at the same time or within a short interval. This substance may be another medicine, a dietary or other type of supplement, a natural product, a herbal tea, or even a food or beverage. The result of an interaction may be an increase, a reduction, or a modification of the medicine effect. The clinical relevance of a drug interaction varies: some result in beneficial synergistic effects, others in potentially serious adverse effects, some partially or completely compromise treatment, and others may have no significant impact. Drug interactions occur for two main reasons: The pathway of the medicine in the body is affected, meaning there is interference with its absorption, distribution, metabolism, or elimination; The two substances - the medicine and the interacting substance - have similar or opposing effects in the body . There are also health conditions, particularly liver and kidney diseases, that affect medicines behavior in the body, altering the expected effect. However, this is not considered a drug interaction because it does not involve another substance. What are the symptoms of a drug interaction The symptoms of drug interaction vary and depend on the medicines and substances involved, as well as each person’s health status. Their severity also varies. Any new or unexpected symptoms after starting a treatment should always be taken seriously by seeking medical or pharmaceutical advice. Among the most common symptoms that may be related to drug interactions (depending on the medicine and substance involved) are: Nausea and vomiting; Changes in bowel habits, including diarrhea; Dizziness, feeling faint, drowsiness, mental confusion; Changes in respiratory rate, blood pressure, and heart rhythm; Abnormal bleeding. If the expected effect of a treatment is not observed within an appropriate time frame, or if there is worsening or decompensation of a previously controlled condition, medical care should be sought. Examples of known drug interactions Medicine – medicine Antibiotics and contraceptives : some antibiotics may reduce the effectiveness of contraceptives, increasing the risk of unplanned pregnancy. Anticoagulants and anti-inflammatory drugs : some anti-inflammatory drugs, such as ibuprofen, may increase the risk of bleeding when taken with anticoagulants. Antacids and antibiotics : some antacids reduce the absorption of certain antibiotics, decreasing treatment effectiveness. Medicine – supplements / teas Hypericum supplements accelerate liver metabolism and may reduce the effectiveness of many medicines, including antidepressants and contraceptives. Valerian tea may enhance the effect of anxiolytics and benzodiazepines. Chamomile, ginger, and ginkgo biloba teas may increase the risk of bleeding when taken with anticoagulants, antiplatelet agents, and anti-inflammatory drugs. Medicine – foods / beverages Grapefruit juice may increase the concentration of some medicines in the blood (such as statins), which is associated with a higher risk of adverse effects. Alcoholic beverages may enhance the sedative effects of benzodiazepines and interact with many other medicines (such as metformin, opioids, among others), potentially causing dangerous effects. A high-fat meal may increase the absorption of some medicines. Measures to reduce the risk of drug interactions Keep an up-to-date medication list and carry it with you when visiting a doctor. In addition to prescribed medicines, this list should include dietary supplements, vitamins and minerals, natural products, homeopathic medicines, over-the-counter medicines, and regularly consumed teas. Always inform your doctor and pharmacist about any supplements or teas you take regularly. Do not assume they are harmless because they are natural products. Avoid consuming large quantities of the same herbal tea ; vary your choices and drink with moderation. Follow the guidance given by your healthcare professionals. Do not change the dose, frequency, or timing of prescribed medicines. Clarify with your doctor or pharmacist how to take a medicine in relation to meals , for example whether it should be taken on an empty stomach, during, or after a meal. Do not stop treatment due to suspected drug interaction without first consulting your doctor or pharmacist. Do not self-medicate , even with over-the-counter medicines, especially if you are taking other medicines regularly. You can always seek advice from a pharmacist or your doctor. The content published in the Health Guide is intended for the general public and is provided for informational and educational purposes only. It does not replace individual medical assessment, diagnosis, or personalized clinical advice from a qualified healthcare professional. The content is produced or reviewed by physicians and other healthcare professionals at Hospital da Luz and is based on the knowledge available at the time of publication or the most recent update, as indicated at the beginning of each item.
