I have two fields and I wish to validate one to depend on the other

This what I would like to happen

if field1 = text1 or text2 or text3
field2 must be populated
If field1 = text4 or text5
Field2 does not need to be populated

Field1 is a drop down menu with 5 text options field2 is an integer I want
field2 dependent on field 1
I have tried the iif function at table level and it didn't work is there
another option?
I'm using 2003