Taking several medications makes remembering when to take each medication harder and increases the risk of adverse drug-drug interactions, particularly when over-the-counter medications are also being taken. Older adults, pregnant people, and children are particularly at risk because they typically require different medications, different doses, or both. Other errors involve drug interactions, in which one drug increases or decreases the effect of another drug. Drug interactions are more likely to occur in people who take many medications. This process is more complex if the person takes other medications and has other diseases, because drug-drug and drug-disease interactions are possible. A standard or average dose is determined for every new medication. Many drugs can be administered orally as liquids, capsules, tablets, or chewable tablets. Because the oral route is the most convenient and usually the safest and least expensive, it is the one most often used. However, it has limitations because of the way a drug typically moves through the digestive tract. For drugs administered orally, absorption may begin in the mouth and stomach. However. Drug-Drug Interactions Drug-drug interactions can involve prescription or nonprescription (over-the-counter) drugs. Types of drug-drug interactions include duplication, opposition (antagonism), and alteration of what the body does to one or both drugs. Duplication When two drugs with the same effect are taken, their side effects may be intensified. Most interactions between a drug and a receptor (a drug binding site on or within a cell) or between a drug and an enzyme (a protein that regulates chemical reactions in the body) are reversible: After a while, the drug disengages, and the receptor or enzyme resumes normal function. Sometimes, the interaction persists (continues after a person stops taking the drug and is sometimes permanent. Drug-Drug Interactions Many people neglect to mention their use of over-the-counter (OTC) medications to their doctor or pharmacist. Medications taken intermittently, such as medications for colds, constipation, or an occasional headache, are mentioned even less often. The drugs then interact with cells or tissues where they produce their intended effects (target sites). This interaction is called selectivity. Selectivity is the degree to which a drug acts on a given site relative to other sites. Relatively nonselective drugs affect many different tissues or organs. Drug-drug interactions can involve prescription or nonprescription (over-the-counter) products. Types of drug-drug interactions include duplication, opposition (antagonism), and alteration of what the body does to one or both drugs. Duplication When two medications with the same effect are taken, their side effects may be intensified. Adverse drug reactions (adverse effects) are any unwanted effects of drugs (medications). There are several different types: Dose-related Allergic Idiosyncratic Dose-related adverse drug reactions represent an exaggeration of the drug's therapeutic effects. For example, a person taking a medication to reduce high blood pressure may feel dizzy or light-headed if the medication reduces blood.