Database Relational Design for Multiple Categories
I second @FrustratedWithFormsDesigner's suggestions for a unique constraint.  See http://office.microsoft.com/en-us/access-help/create-a-constraint-adp-HP003088257.aspx for how to do this in MS Access.

What a Unique Constraint does is state that no two rows can have duplicate data on some portion of the row.  So presumably you want each patient to be in a study only once, so you'd want a unique constraint on PatientsInStudy(participantID, study).

This way each individual can only be given one study id.  Similarly in that design your PatientsInStudy table would have a primary key spanning all three fields, and a second unique constraint on (StudyID, study_code).  This way even if two studies end up with similar semantics or overlapping spaces, the key cannot be reused in the same study.

Otherwise, good.