What's the rule behind the change of the pronunciation of '薬' in '目薬'?


目薬 is pronounced 'めぐすり' while 薬 'くすり'.  
The pronunciation of '薬' change from 'くすり' to 'ぐすり'. What's the rule behind
this change?




This is [rendaku](https://en.wikipedia.org/wiki/Rendaku) (連濁) which commonly
occurs in compounds in Japanese. Unfortunately, in general whether it applies
or not isn't at all predictable. However, there are tendencies including the
well-known [Lyman's law](https://en.wikipedia.org/wiki/Rendaku#Lyman%27s_Law)
which at least tells you where it _cannot_ occur.

