Degraded by the light

Today I have a chemical conundrum for you: My cat was recently prescribed Clavamox (consisting of amoxicillin trihydrate and clavulanate potassium), and the vet warned me to keep the pills in their opaque packaging until their use because they turn brown and disgusting (and presumably become less effective) when exposed to light. I have of course heard of light-sensitive drugs in the form of photodynamic therapy, or in regards to the idea that milk should be protected from light, but I hadn’t run across light-sensitive antibiotics (not sure whether that’s a testament to my poor chemical knowledge or my good health). From a very speedy search, it seems that amoxicillin, and not clavulanate, is the delicate compound in the mix.

So, the question for you all, as I am too lazy (I mean busy. Busy!) to figure it out myself, is: why is amoxicillin light-sensitive?

Catherine (associate editor, Nature Chemical Biology)