DECLARE @Id INT;
    SELECT @Id = id
    FROM sysobjects
    WHERE name = 'TblDrugUse'
IF EXISTS (SELECT * FROM sysobjects WHERE parent_obj=@Id AND xtype='PK')
begin
ALTER TABLE TblDrugUse   DROP CONSTRAINT PK_TblUseDrug
 ALTER TABLE TblDrugUse ALTER COLUMN Use_Code VARCHAR(9) NOT null
 ALTER TABLE TblDrugUse ADD CONSTRAINT PK_TblUseDrug Primary Key (Use_Code)   
end