The web is loaded with good reports about Melatonin as a natural
sleeping aid.  A correct answer to this question is a link to some
documented studies or credible evidence of any negative long term
effects associated with prolonged use of Melatonin.

Dear tibiaron-ga
It seems that the effects of the long-term use of melatonin and how it
interacts with other medications or supplements, is still largely
unknown. There are a number of studies currently underway.
I first refer you to five sites which all state this. There is also
some discussion of the short-term side effects. I will then refer you
to four studies where comments have been made on long term side
effects.

"Long-term: Unknown. Caution is appropriate because effects of
hormones may not become apparent for years"
1993
http://www.websciences.org/sltbr/melfaq.htm

"Does melatonin have side effects?
Some people who have taken melatonin have reported sleepiness,
headache, a "heavy-head" feeling, stomach discomfort, depression or
feeling hungover. Further study is needed to find out more about
melatonin's side effects, especially delayed or long-term effects. For
example, we don't know if melatonin causes problems when taken with
other medicines. We also don't know how melatonin may affect
diseases."
http://www.getpharma.com/product.asp?productid=51


"Precautions: There are no known serious side effects to regulated
melatonin supplementation. Some people may experience vivid dreams or
nightmares. Overuse or incorrect use of melatonin could disrupt
circadian rhythms. Long-term effects have not been well studied. In
rats, melatonin decreases T4 and T3 uptake levels.
Melatonin can cause drowsiness if taken during the day. If morning
drowsiness is experienced after taking melatonin at night, reduce
dosage levels. In some cases of depression, daytime doses of melatonin
can increase depression.
May be contraindicated for those with autoimmune disorders and immune
system cancers (e.g., lymphoma, leukemia).
Because melatonin suppresses corticosteroid activity, those who are
taking corticosteroids for anti-inflammatory or immune suppressive
purposes (e.g., transplant patients) should exercise caution with
melatonin supplementation.
Melatonin could interfere with fertility. It is also contraindicated
during pregnancy and lactation.
Lack of sleep and insufficient exposure to darkness may suppress
natural production of melatonin."
http://www.wellfx.com/InfoBase/vitamin_melatonin_.htm


"The bottom line is that the "jury is still out" on long-term use of
this product, particularly in younger children."
Several observations on the long term effects with links to various studies.
2003
http://www.medscape.com/viewarticle/448467

Information letter which again states that the ?jury is still out?
http://www.berkeleywellness.com/html/ds/dsMelatonin.php


The following are abstracts from various studies which have been published.

"Safety in melatonin use" 2001
Review of adverse effects of MLT consumption in humans
307 articles were elicited and 9 were related to MLT adverse effects
"The adverse reactions were: one patient with autoimmune hepatitis,
one case of confusion due to MLT overdose, one case of optic
neuropathy, four subjects with fragmented sleep, one psychotic
episode, one case of nistagmus, four cases of seizures, one case of
headache and two cases of skin eruptions. CONCLUSIONS: Attention
should be paid on the necessity of enquiring about the drugs that
patients are taking, because this product is not harmless for health."
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11602091&dopt=Abstract&itool=iconabstr

Reproductive safety of melatonin: a "wonder drug" to wonder about
1997
..."There is little information on the potential adverse effects of
melatonin ingestion in humans..(..) ..
The decision to ingest melatonin should be preceded by careful
consideration of the expected benefits as well as the potential costs
of treatment, with recognition of the fact that there has been
exaggeration of the benefits and little attention paid to the
potential costs in most discussions of this issue to date.."
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9406045&dopt=Abstract&itool=iconabstr


Toxicology of melatonin 1997
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9406047&dopt=Abstract&itool=iconabstr

Is melatonin associated with the development of autoimmune hepatitis? 1997
"The temporal relation observed between melatonin use and the
development of autoimmune hepatitis raises the possibility that the
drug might be involved in the pathogenesis of this patient's
autoimmune disease."
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9412927&dopt=Abstract&itool=iconabstr

Melatonin and Sleep Disturbances 1999  Some references to long-term effects
http://www.psychiatrictimes.com/p990370.html


I hope this answers your question. If it does not, or the answer is
unclear, then please ask for clarification of this research before
rating the answer. I shall respond to the clarification request as
soon as I receive it.
Thank you
answerfinder


Search Strategy
Melatonin "side effects" "long term"
://www.google.com/search?q=Melatonin+%22side+effects%22+%22long+term%22&hl=en&lr=&ie=UTF-8&oe=UTF-8&start=0&sa=N

Great job, thanks.