¿What proof exists that methylphenidate is a drug that can cause addiction or that it has a high abuse potential that justifies the present day control on its sale?

None; many medical articles endorse this statement. The present day standpoint, widely accepted by physicians who have knowledge and experience in the subject, is that far from producing or favoring addiction, methylphenidate protects ADHD patients from that risk. The studies have been done on stimulant drugs; as methylphenidate has been the most widely used stimulant drug in the last 30 years, the results apply especially to it.

There are at least twelve studies in the academic medical literature that endorse this, against one single study —which has been widely criticized due to its defective methodology— which states otherwise. This is thoroughly discussed by Dr. Russel Barkley — a pychologist who is an authority in the field of ADHD— in an article published in Pediatrics in January 2003:

Russell A. Barkley, Mariellen Fischer, Lori Smallish and Kenneth Fletcher. Does the Treatment of Attention-Deficit/Hyperactivity Disorder with Stimulants Contribute to Drug Use/Abuse? A 13-year Prospective Study. Pediatrics, Vol. 111 n.º 1. January 2003 (pages 98 and 99 are especially pertinent).
Link to the full text article:
http://pediatrics.aappublications.org/cgi/content/full/111/1/97    
Link to an abstract of the article:
http://pediatrics.aappublications.org/cgi/content/abstract/111/1/97

Dr. Joseph Biederman from Harvard Medical School and the Massachusetts General Hospital —a recognized medical authority in the field of ADHD— demonstrated, in two excellent articles published in 1999 and 2003, that stimulant drugs protect ADHD patients from addiction.

Joseph Biederman, Timothy Wilens, Eric Mick, Thomas Spencer and Stephen V. Faraone. Phamacotherapy of Attention-deficit/Hyperactivity Disorder Reduces Risk for Substance Use Disorder. Pediatrics, Vol. 104, n.º 2, August 1999.

Link to the full text article:
http://pediatrics.aappublications.org/cgi/content/full/104/2/e20      
Link to an abstract of the article:
http://pediatrics.aappublications.org/cgi/content/abstract/104/2/e20  

Joseph Biederman. Pharmacotherapy for Attention-Deficit/Hyperactivity Disorder (ADHD) Decreases the Risk for Substance Abuse: Findings from a Longitudinal Follow-Up of Youths With and Without ADHD. Journal of Clinical Psychiatry. 2003. Vol. 64 (suppl 11).
Link to the full text article:
http://www.psychiatrist.com/supplenet/v64s11/v64s1101.pdf
Link to an abstract of the article:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=14529323&query_hl=10&itool=pubmed_docsum    

Dr. Nora Volkow, Director of the National Institute on Drug Abuse, NIH, and the distinguished neuroscientist Dr. James Swanson, have shown that, after oral administration,  methylphenidate's slow entrance to the brain and its even slower exit set it apart from drugs which produce addiction.

Volkow, N.D. and James M. Swanson. Variables That Affect the Clinical Use and Abuse of Methylphenidate in the Treatment of ADHD. American J. Psychiatary. 160:1909-1918. 2003.
Link to the full text article:
http://ajp.psychiatryonline.org/cgi/content/full/160/11/1909
Link to an abstract of the article
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=14594733  

There is evidence that the abuse potential of methylphenidate is lower in ADHD people, probably due to the increased number of dopamine transporters they have.

Kollins, Scott H. Comparing the Abuse Potential of Methylphenidate Versus Other Stimulants: A Review of Available Evidence and Relevance to the ADHD Patient. J Clin Psychiatry. 2003;64(suppl 11):14-18.
Link to the full text article (especially page 17):
http://www.psychiatrist.com/supplenet/v64s11/v64s1103.pdf  
Link to an abstract of the article:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=14529325&query_hl=12&itool=pubmed_docsum    

The abuse potential of methylphenidate would be limited to the abnormal routes of administration, like intravenous injection and inhalation.  

James M. Swanson and Nora D. Volkow. Serum and brain concentrations of  methylphenidate: implications for use and abuse. Neurosci Biobehav Rev. 2003 Nov,27(7):615-21.
Link to an abstract of the article:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=14624806

The phenomenon known as sensitization in experimental animals has never been shown to happen in humans. There is no evidence that children treated with methylphenidate have a higer risk of addiction in adult life. Researchers from NYU Child Study Center have studied this aspect thoroughly:  

Klein R. and Mannuzza S. Is there stimulant sensitivity in children? J Atten Disord. 2002;6 Suppl 1:S61-3
Link to an abstract of the article:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=12685520 

Finally, besides all the evidence that has been shown, it is reasonable to think that if methylphenidate helps people with ADHD normalize their lives —a fact demonstrated beyond any doubt— the risk of drug addiction, due to repeated frustrations and decreased likelyhood of success, must necessarily be lower in those who are being or who have been treated with this drug.

This translated text has been extracted from:" Reunión con la Ministra de Salud sobre la venta del Ritalin", published in APDA’s newletter nº 5, September 19, 2004, which can be read on this website:  http://www.deficitdeatencionperu.org/reunion%20ministra.htm
Modified in September2006.
Dr. Filomeno’s e-mail is: armandofilomeno@telefonica.net.pe