EPQ Topics around DentistryWatch
People with depression often have really low mood and hence struggle to complete tasks that are 'normal' such as brushing their teeth. In turn, that leads to poor oral hygiene.
Maybe you could also explore ideas on how to help people with mental health problems battle against this, i.e. using gamification (an area I focused on in my EPQ).
EPQ sources don't need to be directly related, linking them together is amazing too. Getting a source on people who are depressed having lower energy and not brushing your teeth leading to poor oral hygiene is good but you just need to make sure you write how they LINK.