This is just interesting and I figured to mention it because many have no doubt heard of this. If you have not, now you have the full explanation without hearing the first part without the explanation and having to wonder why. These two drugs do not convert back and forth in the body, the above example is simply illustrative of why you may have heard or read before that these two drugs are similar. Only in structure. In action they are very different (as many of you who have used each of these drugs, no doubt knows). But now you know structure-wise. And if you have heard this before, now you know exactly why they are structurally similar.