Hi @mednlx
I found your question interesting and I hope you don't mind if I try to answer. ICD codes for long term use of a drug do not generally have a specific time or dosage requirement, rather it's based on how and for what they are used. The guidelines on how to select these codes are in the coding guides, for example:

ICD9 - https://www.ama-assn.org/sites/ama-assn.org/files/corp/media-browser/public/cpt/icd9cm_coding_guidelines_08-09_sm_0.pdf:

"Assign a code from subcategory V58.6, Long-term (current) drug use, if the patient is receiving a medication for an extended period as a prophylactic measure (such as for the prevention of deep vein thrombosis) or as treatment of a chronic condition (such as arthritis) or a disease requiring a lengthy course of treatment (such as cancer). Do not assign a code from subcategory V58.6 for medication being administered for a brief period of time to treat an acute illness or injury (such as a course of antibiotics to treat acute bronchitis)."

And ICD10 - https://www.cms.gov/files/document/fy-2024-icd-10-cm-coding-guidelines-updated-02/01/2024.pdf:

"Assign a code from Z79 if the patient is receiving a medication for an extended period as a prophylactic measure (such as for the prevention of deep vein thrombosis) or as treatment of a chronic condition (such as arthritis) or a disease requiring a lengthy course of treatment (such as cancer). Do not assign a code from category Z79 for medication being administered for a brief period of time to treat an acute illness or injury (such as a course of antibiotics to treat acute bronchitis). "

Hope this helps.

Mary