It's because the Text Box is Null.

    If CB Then
        cbModified = Nz(cbModified, 0) + 1
        Me.Refresh
    End If

Null
NZ Function

You also may want to change your naming convention to avoid reserved words. See https://599cd.com/glossary/access/reserved/

You may also need to subtract 1 if the box is unchecked, or if the box is unchecked and rechecked, the value would have risen by 2, and furthermore if it was clicked more times.  Utilizing Kevin's Nz idea, perhaps:
cbmodified = Nz(cbmodified, 0) + IIf(CB=True,1,-1)

Thanks Kevin for your swift and accurate answer.   I had thought of that but I didn't formulate the nz correctly   I placed it on both sides of the = and that caused vba to gag.   Your solution worked fantastically.   Many thanks.

Thanks Brian.  That's why the table was table and the form was form1.

However the real script as applied is as follows:

In my global module

Public Sub updatetotalcb() 'increment counter field  prev value is TempVars!currenttotalcb.Value
If Screen.ActiveControl = -1 Then
    Forms!onboard!OnboardingStepsSubf.Form!totalcb = Nz(Forms!onboard!OnboardingStepsSubf.Form!totalcb, 0) + 1
End If
If Forms!onboard!OnboardingStepsSubf.Form!totalcb > TempVars!currenttotalcb.Value Then MsgBox "meow" & vbNewLine & "email trigger."

End Sub


Then on my checkboxes:

Private Sub SentToDrugScreen_AfterUpdate()
updatetotalcb
Me.Refresh
End Sub

Private Sub SentToDrugScreen_BeforeUpdate(Cancel As Integer)
TempVars!currenttotalcb.Value = totalcb.Value

End Sub

Thanks for having a good eye on my naming laziness.  you'll keep me disciplined  :)

fortunately I don't need to subtract from the total on this when a checkbox is unchecked.   the email trigger will go off simply if the totalcb field is > the previous button.