MySQL not using indexes with WHERE IN clause?

I'm trying to optimize some of the database queries in my Rails app and I have several that have got me stumped. They are all using an IN in the WHERE clause and are all doing full table scans even though an appropriate index appears to be in place.

For example:

SELECT `user_metrics`.* FROM `user_metrics` WHERE (`user_metrics`.user_id IN (N,N,N,N,N,N,N,N,N,N,N,N))

performs a full table scan and EXPLAIN says:

select_type: simple
type: all
extra: using where
possible_keys: index_user_metrics_on_user_id  (which is an index on the user_id column)
key: (none)
key_length: (none)
ref: (none)
rows: 208

Are indexes not used when an IN statement is used or do I need to do something differently? The queries here are being generated by Rails so I could revisit how my relationships are defined, but I thought I'd start with potential fixes at the DB level first.