Without more information (like how the two variables are formatted, etc), its a little difficult to give an exact answer for you situation.
But I created a dummy table where both fields are character formatted.
DATA WORK.HAVE;
FORMAT rxcode_1 $10. erxname1 $20. ;
INFORMAT rxcode_1 $10. erxname1 $20. ;
INPUT rxcode_1 erxname1;
INFILE DATALINES DLM='|' DSD;
DATALINES;
333|lexapro
333|lexapro
9347410|
333|lexapro
;
With this table, the following logic can be used to fill the blank value where the rxcode_1 = '9347410':
DATA WORK.WANT;
SET WORK.HAVE;
IF rxcode_1='9347410' THEN erxname1='citalopram';
RUN;
Final output table results:
rxcode_1 erxname1
333 lexapro
333 lexapro
9347410 citalopram
333 lexapro
Hope this helps.