Hi there I'm having issues trying to identify the minimum value of date of dispensation for each drug in the data set. Here's a sample data set and I've also attached a PDF copy to make it easier to read.

Rcpt_Anon_ID str8 DRUG_DIN double DSPN_AMT_QTY long DSPN_DATE double DSPN_DAY_SUPPLY_QTY str10 Prscb_Anon_ID str8 SUPP_DRUG_ATC_CODE
"000009106" "02261731" 3 18169 84 "390203576" "G03AA12"
"000009106" "02405628" 90 19971 90 "531829076" "C10AA07"
"000009106" "02353377" 180 19031 90 "277246846" "A10BA02"
"000009106" "02282445" 30 19400 30 "277246846" "N05CF01"
"000009106" "02405628" 90 20765 90 "277246846" "C10AA07"
"000009106" "02353377" 180 19838 90 "130332376" "A10BA02"
"000009106" "02405628" 90 21186 90 "385893456" "C10AA07"
"000009106" "02282445" 90 20974 90 "277246846" "N05CF01"
"000009106" "02353377" 180 19115 90 "277246846" "A10BA02"
.
Here's the code I ran to try and get a minimum date value for this data set

g opioid=substr(SUPP_DRUG_ATC_CODE,1,4)=="N02A"

. by Rcpt_Anon_ID: egen o_date = min(opioid) if opioid ==1

. format o_date %tdD_m_Y

. display o_date

after running the date, I'm not getting any values in return.

Any help would be great, thank you