Drug interactions happen when the combination of a drug with another substance causes different effects than either would individually. Foods, herbal supplements, legal drugs, and illicit substances can all cause drug interactions. Most drug interactions fall into two categories. Some take place when two substances effects influence each other directly. In other cases, one substance affects how the body processes another, like how it is absorbed, metabolized, or transported around the body. Blood thinners and aspirin, for example, have similar effects that become dangerous when combined. Both prevent blood clots from forming blood thinners by preventing the formation of the clotting factors that hold clots together, and aspirin by preventing blood cells from clumping into groups that become clots. Individually, these effects are usually safe but taken together, they can prevent blood clotting to a dangerous extent, possibly causing internal bleeding. While blood thinners and aspirin are generally harmless when taken individually, interactions, where one substance exacerbates the effects of another, can also take place between drugs that are independently harmful.