"Whoever fights monsters should see to it that in the process he does not become a monster. And if you gaze long enough into an abyss, the abyss will gaze back into you." - Friedrich Nietzsche
Tokyo Dark takes a very Lovecraftian approach to sanity. If you gaze into the abyss it'll not only gaze right back at you, it will tear into your psyche and leave you a gibbering mindless wreck.
As Detective Itō investigates the otherworldly passages, tunnels and caves deep below Toyko she may encounter distortions or living shadows. The only sensible action to take is to run and find somewhere to hide, the presence of a distortion in reality will lower Itō's sanity and if not treated eventually her mind will snap.
Having lower sanity will affect your conversation choices, the way people react to you and you'll see things, whether they are hallucinations or not are for you to decide.
To manage her sanity Detective Itō will have to self-medicate by taking pills, but that comes at a cost. Pills raise her SANITY attribute but lower INVESTIGATIVE SKILLS. She no longer sees the things and can walk through an otherwise terrifying room with a smile on her face. But whilst perfectly sane she has also dulled her senses and will not notice as many clues and interactive objects around her. The relationship between these attributes will lead to some interesting choices, how much of Itō's mental health are you willing to risk to solve the case?
The pills make the terrifying visions go away and keep you sane, but they also dull your mind and slow your thoughts, do you take the medication?