my son is almost 3 years now. he is speaking very little. just words and no sentences and i feel that he even does not understands every thing i say.
what can be the possible cause and what should be the right approach to deal with this.
Please share if you have faced similar problems. Now i am really getting more and more tensed day by day.

He might just have a speech or learning impediment that's not necessarily adhd. there are tons of books about childhood adhd! if you are getting that worried, I would take him to a doctor. But I think you could easily find answers at the library. I hope things get better.

It sounds more like a developmental disorder than ADHD.

I'd see a physician.
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Related to ADHD
Watch few minutes video with Dr. Mary Ann Block to find answers you are looking for.

cchrint.org/2009/09/15/no-mor…

Dr. Mary Ann Block
"Because of my medical training, my goal as a physician is to look for and treat the underlying conditions causing the patient’s problem, rather than just covering up those symptoms with drugs. I have seen and treated thousands of children from all over the United States, who had previously been labeled ADHD and treated with amphetamine drugs. by taking a thorough history and giving these children a complete physical exam as well as doing lab tests and allergy testing, I have consistently found that these children do not have ADHD, but instead have allergies, dietary problems, nutritional deficiencies, thyroid problems and learning difficulties that are causing their symptoms. All of these medical and educational problems can be treated, allowing the child to be successful in school and life, without being drugged."

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I would recommend you reading following booklet for parents

fightforkids.org/Psychiatric_…

and following less than 2 minutes video about psychiatry diagnosis

youtube.com/watch?v=n3JQ8OVHV…

…and

Psychiatry – NO SCIENCE-NO CURES (4:54min) Do psychiatrists and by them prescribed drugs help people?
youtube.com/watch?v=UHu7Ik361…
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you can also google or yahoo "adhd fraud" – you get good links like
adhdfraud.org/
Articles, essays, and other information pertaining to the fraud of Attention Deficit Hyperactivity Disorder (ADHD)–Compiled by Dr. Fred Baughman

Fred A. Baughman Jr., MD has been an adult & child neurologist, in private practice, for 35 years.