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?
1 Answer
This is 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 which at least tells you where it cannot occur.