SPOILER ALERT!  I tried to keep it vague but this does contain some spoilers.

Let’s start with the two main branches of the story.  Which side you go down is determined by whether or not you listen to Tomlinson or kill him when you’re in his office.  To get the first branch, just select the “End it” option at either one of the prompts.  To get the second branch, select “Listen” and then “Final Warning”.  Once you’re on a branch, most of the endings are determined by which medicine you decide to take earlier in the story.  However, if you go down the second branch, whether or not you decide to complete the ritual with Emmeline determines which ending you get.  If you complete her ritual, you will be locked into the 7th ending regardless of which medicine you took.  Conversely, if you say “No” to her, you can get one of the 3 medicine endings, depending on which you took earlier.  That should cover all the endings.