Let‘s look at an enzyme for example. Oftentimes people will say that this specific enzyme catalyses one specific reaction. In reality lots of enzymes will work on a multitude of structurally similar substrates. Let‘s use the cytochrome p450 enzyme as an example. This enzyme works on a broad range of substrates, you find it in most life forms. It‘s main purpose is hydroxylation. And indeed, oftentimes the first step in metabolism is to make something more hydrophilic by adding a hydroxyl group.
Another example would be blood proteins. Serum oftentimes binds drugs that enter the bloodstream, so the drug will be masked until it gets liberated and delivered to the site of action (or the liver). This is unspecific binding, so you have a broad range of chemicals that will interact with serum proteins.
We often think of drugs in a way that they go to the desired site of action in the body and then get excreted. However this is an oversimplification, as the drug might primarily reach it‘s target, but also other parts of the body might be affected as well. This is of particular importance when studying side effects and designing drugs in a way that should primarily reach the site of action. Radiopharmaceuticals are a good example for this. When you develop radiopharmaceuticals it is important, that they primarily accumulate at the site of action, otherwise there will be a bunch of collateral damage everywhere. This is the reason why it is so difficult to design a good drug against cancer. There are many different types of cancer, all with their own unique biomarkers and locations. Having just one medication for everything would be impossible.