What's the plural form of phenobarb? Here's the word you're looking for.
Answer
The noun phenobarb can be countable or uncountable.
In more general, commonly used, contexts, the plural form will also be phenobarb.
However, in more specific contexts, the plural form can also be phenobarbs e.g. in reference to various types of phenobarbs
or a collection of phenobarbs.