If it is a Text field then why wouldn't this code work?
Code:
Private Sub change_smoking_AfterUpdate()
If Me![change smoking] = "8" Then
Me![Ceased smoking] = ""
Me![Ceased smoking].Enabled = False
Me![ReducedSmoking] = ""
Me![ReducedSmoking].Enabled = False
Me![IncreasedSmoking] = ""
Me![IncreasedSmoking].Enabled = False
Else
Me![Ceased smoking] = ""
Me![Ceased smoking].Enabled = True
Me![ReducedSmoking] = ""
Me![ReducedSmoking].Enabled = True
Me![IncreasedSmoking] = ""
Me![IncreasedSmoking].Enabled = True
End If
End Sub