It sounds as though you really need 3 tables: one for the patients, one for
the antibiotics, and one that represents the resolution of the many-to-many
between patients and antibiotics. (the primary key of this third table would
be the combination of the patient id and the antibiotic id).
This is very similar to the situation between Products and Orders in the
Northwinds database that comes with Access: each Product can appear on one
or more Orders, and each Order can contain one or more Products. To handle
that many-to-many, the Order Details table was introduced.
Take a look at the Orders and Orders Subform forms in that database as well.
The Orders form is bound to a query based on the Orders table, while the
Orders Subform is bound to a query based on the Order Details table. So that
it's not necessary to type in product names, the Orders Subform includes a
combo box whose RowSource is the Products table.

Signature
Doug Steele, Microsoft Access MVP
http://I.Am/DougSteele
(no e-mails, please!)
> I'm not sure I can even explain this simply enough to get a response but
> I'll
[quoted text clipped - 24 lines]
> I hope this makes some sort of sense. I'm a newbie at database design etc
> so if anyone can help please put it in simple terms!! Many thanks