A drug interaction occurs when one medication affects the activity of another when both are taken together. This can lead to increased side effects, reduced effectiveness, or unexpected reactions. For example, if someone takes a blood thinner and a pain reliever that also affects blood clotting, it could increase the risk of bleeding. It's important to inform your doctor or pharmacist about all the medications you are taking, including over-the-counter drugs and herbal supplements. They can help identify potential interactions and adjust your treatment plan to ensure safety and effectiveness. Always follow their guidance to avoid complications.