How does Efudex work?

I think it's pretty fascinating how Efudex works - even though I don't have the background in cell biology to fully understand the process. Efudex is just one of the brand names for 5-Flourouracil. (Carac is another brand.) As the name implies, flouro-uracil is a modified form of the molecule uracil with an extra flourine atom stuck on. And uracil is one of the basic building blocks of RNA. It also plays a key role in the cell's synthesis of DNA.

Flourouracil

Although the full description of what happens is apparently quite complicated, the essence appears to be that the flourouracil molecule is similar enough to uracil that the cell incorporates flourouracil in processes where it would usually incorporate uracil. So, as the cell prepares to divide, it builds the intermediate molecules that lead to new RNA and DNA using flourouracil instead of uracil. But the molecules built using flourouracil don't function properly! That additional flourine atom, riding along like the soldier inside a Trojan Horse, spoils the operation. And since the cells have evolved to be good team players, when the cell somehow recognizes that something is seriously wrong with the RNA and DNA that it is producing, it commits suicide (apoptosis) rather than hurt the team (the whole person) by spawning defective offspring.

Since the process primarily affects cells that are dividing, it preferentially kills cells that are dividing more rapidly than normal - including cancerous and pre-cancerous cells (including actinic keratoses). The result is that the areas that contain lots of those types of cell become inflamed and eventually shed the dead skin cells through peeling, and the dead cells are replaced by new, undamaged cells.