" The effects of lyrium addiction for templars include paranoia, obsession, and dementia.[24] Symptoms of lyrium hunger pangs include fatigue, forgetfulness, an unquenchable thirst, and cold hands.[25] Over time, templars grow disoriented, incapable of distinguishing memory from present, or dream from waking. They frequently become paranoid as their worst memories and nightmares haunt their waking hours.Cullen Rutherford states that templars also lose their memories to prolong imbibing of lyrium. It starts small at first- a misplaced item or words to a song- but more fades away over time. Mages have additionally been known to suffer physical mutation.[11] According to Vivienne, anything that increases the capacity of a mage's magic (such as lyrium) creates a damaging effect on the body. For mages, this is known as mana imbalance. Some adverse effects of mana imbalance may include conscious dreaming in the Fade (such as those experienced during the Harrowing), periods of dizziness, and hearing voices. Unlike templars, mages are able to recuperate from the adverse effects.
Justice's unique ability to equip the lyrium ring implies that spirits are resistant to lyrium's toxic effects."
http://dragonage.wikia.com/wiki/Lyrium
Also, since this is still the spoiler free section, in Trespasser