Rare is a synonym of Unusual. Unusual is a different concept from Normal. Normal is a synonym of Convention. Convention is a type of Station. 
Pill can be used for Headache. Pill is a type of Dose. 
Rare <--Synonym--> Unusual <--DistinctFrom--> Normal <--Synonym--> Convention --IsA--> Station 
Headache <--UsedFor-- Pill --IsA--> Dose 