Oft used, rarely defined. Okay, maybe 'oft' isn't the right word - Google turns up just 31,900 hits, and it has only appeared in few modern dictionaries in the past five years.
Narcomania is a psychophysical disease in which one is dependent on narcotics. Due to the lack of a stringent medical or legal definition, it may be used in slightly different senses; a psychological dependence on any substance that reduces pain, and acute craving for narcotics, etc. Informally, it just means "you're taking drugs, and it's a problem".
It seems to be a legal term rather than a medical one, but I haven't been able to find a good legal definition. Drug addiction is a much more common term.
If you were to put a little accent mark over the i, you would have the Spanish word for drug addiction.
This is very often coupled with/compared to toxicomania.