I have to update a table B based on table A. Table A has got two field

The code is:
db.Execute ("UPDATE A LEFT JOIN B ON A.MedicineID=B.MedicineID SET B.Medicines = A.Medicines ")

but the problem is , the above code doesn't work.
When I ran the same sql query manually by creating a test query,
the updated table B value is :
MedicineID = 0
Medicines = test

whereas, the table A value is
MedicineID = 20, Medicines =test

Can anyone please help me on this query, or is there is another easy way to do it.

Thanks in advance, and
Merry Christmas !!!