First, the family story of your GG/Aunt being asthmatic and died of an overdose, by itself, may or may not be true depending on family lore. But, if she was an inpatient at the North Idaho Asylum, that gives a much less value to this family lore. As an inpatient, her medication would have been limited to that given by doctors/ nurses/ medical staff.
Most States are highly protective of the rights of those voluntarily and involuntarily admitted into mental hospitals. Some of these States protect this information even after death. The verbiage of the statutes in Idaho indicate these laws extend to after death. Only until recently were records made available to identify those patients who were buried in State Hospital Cemeteries in Washington State.
This is the primary Idaho statute concerning mental health patent records:
Title 66: State Charitable Institutions;
Chapter 3: Hospitalization of Mentally Ill http://legislature.idaho.gov/idstat/Title66/T66CH3.htm
66-348: Disclosure of Information http://legislature.idaho.gov/idstat/Title66/T66CH3SECT66-348...
Basically says: If the person or his attorney consent; or if necessary to carry out ...(laws)... or if the court directs disclosure is necessary...
Title 9: Evidence
Chapter 3: Public Writings
9-340C. Records Exempt from Disclosure... Paragraph 6http://legislature.idaho.gov/idstat/Title9/T9CH3SECT9-340C.h...
Her death certificate if issued by the State Asylum, or if identifies mental state, it is NOT releasable. UNLESS, you obtain a court order for release. Mere curiosity has been shown to not be a valid reason for release of confidential information. If you could prove a need to have the medical information to protect yourself or family, the court "might" take interest. If they did, they would review your petition, then review any and all documents subject to release to verify a legitimate need for release. But, in this case, I don't believe any medical history would be released. The court "might" release information as to the place of burial.
IF she was NOT a patient, you can order a NON-Certified copy of her death certificate for $14.00. There are other restrictions for a CERTIFIED copy:http://healthandwelfare.idaho.gov/Health/VitalRecordsandHeal...
You could apply for her death certificate, if it is falls under the Idaho Statute for protection, ALL you can be told is there is no record. Legally, they can not tell you she was a patent, or the DC is protected.
If her death was due to overdose, or she had been an inpatient, the family probably would not have identified this in an obituary. You could try to obtain one, there is a volunteer here:http://freepages.genealogy.rootsweb.ancestry.com/~obitl/voli...
I base my comments above on my experience with Washington State and California attempts (CA for a friend) to obtain records. At one point, I was sent an impressive stack of papers to apply for court order for disclosure for Washington State. These mental health records are much like adoption records. They not only protect the person from disclosure; but also the family. I doubt you would like someone to obtain mental health records of your aunt to embarrass you.