If you right click any where on the User Interface, the interface will change to the minimal mode which is 3 small lines located at the bottom of the screen. The lines represent your health, mana and stamina levels. They are tiny at higher resolutions so it may not be evident what they are. To return the UI to full size, right click any where on the red. blue and yellow bars at the bottom of the screen.
You should LEFT click the potion icon on the UI which opens a widow showing all of your potions. Left click the potion you wish to use on the potion window during combat. It is better to link an F key (F-1 to F-12) to the potions which allows you to simply hit the F key to use the potion.
See the attached image . . . =) . . . This is for skills but Potions link the same way, see green text
Edit: You cannot drink potions while you have the CTRL key pressed. CTRL key causes your character to attack the nearest enemy but while you are holding it down, you can't drink potions, got killed alot before I figured that out . . . =)
Edit: Also you CANNOT run away while the CTRL key is pressed . .