ICD-10-CM > Z00-Z99 > Z77-Z99 > Z91 > Z91.1 > Z91.13 Z91.13 Patient's unintentional underdosing of medication regimen Non-Billable/Non-Specific Code First underdosing of medication (T36-T50) with fifth or sixth character 6 Codes Z91.130Patient's unintentional underdosing of medication regimen due to age-related debilityZ91.138Patient's unintentional underdosing of medication regimen for other reason