Was Edgar Allan Poe mentally unstable or under the influence of drugs or alcohol, and if so how did that influence his work?
Edgar Allan Poe admitted to using opium, but this was a prominent treatment for medical problems at the time. There was no evidence that he was addicted to it (http://edgarallan2002.tripod.com/misc.html).
Poe was, however, a known alcoholic. He lost a couple of jobs because his employers threatened to fire him if he did not stop drinking. Even under this pressure, he still could not give it up (http://edgarallan2002.tripod.com/misc.html). Poe used alcohol to deal with his problems. His wife (also his cousin) popped a blood vessel and was sick with tuberculosis for 5 years before she finally died. After her death, his drinking became even more of a problem (http://www.kirjasto.sci.fi/eapoe.htm). For more information on Poe's life, see this video:
Poe’s sister Rosalie was insane. Since diseases such as schizophrenia are genetic, it is possible that Poe might have also suffered from the same thing. He married his 13 year old cousin. Since that is abnormal, it could be a sign that something was wrong with Poe. Poe also attempted suicide and wrote about some of his suicidal thoughts in letters. Kay Redfield Jamison, an expert on bipolar disorder, has looked at some of Poe’s letters and speculates that he was bipolar. She says that during mania, people with bipolar disorder are likely to be very creative, so this is where he would have done all his writing. The suicidal thoughts and attempts would have come into play during the depression. Someone asked Poe about his madness and he said, “"Men have called me mad; but the question is not yet settled, whether madness is or is not the loftiest intelligence -- whether much that is glorious -- whether all that is profound -- does not spring from disease of thought -- from moods of mind exalted at the expense of the general intellect” (http://curiosity.discovery.com/topic/perception/mad-genius3.htm). Poe means that if he was mentally ill, it would have added to his creativity. If there was something wrong with Poe, he did not seek treatment because treatment in those times for mental disorders was brutal and experimental, such as cutting holes in the skull to let evil spirits out (http://edgarallan2002.tripod.com/misc.html).
If Edgar Allan Poe was bipolar or schizophrenic, that would explain some of his stories. Schizophrenics hear voices in their heads and think they’re real like how the narrator in the Tell Tale Heart hears the thumping of a dead man’s heart. His depressed moods would explain why his stories and poems are always so sad. Because there was limited knowledge about mental disorders at this time in history, it would be hard for Poe to have seen inside of his characters’ minds unless he too thought the same way. So, while there is no definitive answer there is a high possibility that Poe was indeed mentally unstable.