I would use VBA to do this. First, create a procedure that determines whether or not the field shoule be enabled, Something like this:
Code:
Sub FieldEnabling()
If Me.Past_Medical_History = True Then
Me.Past_Med_History_Details.Enabled = True
Else
Me.Past_Med_History_Details.Enabled = False
End If
End Sub
Then, call this code from the "Current" event of the Form (so that is is run every time you open the Form or go to a new record:
Code:
Private Sub Form_Current()
Call FieldEnabling
End Sub
and also call it from the AfterUpdate event of your Yes/No field (so it gets updated if that field changes):
Code:
Private Sub Past_Medical_History_AfterUpdate()
Call FieldEnabling
End Sub