[amintobello]

When i was surfing at wikipedia i realized that nearly all of drugs i have seen have at least one benzene ring and i looked all of drugs at wikipedia.There is nearly %80 of drugs have at least one benzene ring.
So there is the question:
Why all of these drugs (from depressants to stimulants) have benzene ring or rings what is so important about benzene?

[Arkcon]

Your question is very general, and its hard to formulate a simple answer.  Something I've noticed is pharmaceuticals often (but not always) contain multiple fused benzine rings, often with nitrogen atoms in the ring.  Something to help you understand it is how these drugs interact with the body, they "trick" an enzyme, or maybe a cellular protein receptor into doing or not doing "something."  You want a molecular interaction between the drug and the protein.  Perhaps one of the most important way for the interaction to occur is pi-pi interaction between the aromatic groups in the drug and the protein in question.

http://en.wikipedia.org/wiki/Pharmaceutical_drug

[amintobello]

I read the link and saw that Wikipedia says:
A psychoactive drug, psychopharmaceutical, or psychotropic is a chemical substance that crosses the blood–brain barrier and acts primarily upon the central nervous system where it affects brain function...
so is benzene(or similars) important for crossing blood-brain barrier?if so how?if not what is the function 

[Arkcon]

Umm ... no, I meant for you to review that web page for all the different types of medicines there are, and see that many have similar structures, multiple fused heteroaromatic rings.  Thinking benzene is the reason for psycho-activity is oversimplifying the problem, and confusing you.

[Nosterius]

Look at Derek Lowe's september 13th comment in his blog:

http://pipeline.corante.com/archives/drug_industry_history/

He cites a publication about a 50 years retrospective of medicinal chemistry, which can be found there:

http://pubs.acs.org/doi/abs/10.1021/jm200504p

It offers a good explanation of the inclusion of aromatic moieties in meds.

[amintobello]

I dont think benzene is the reason  and i cant think that because there is drugs doesn't have benzene ring but i just thought that benzene could have a relation if lots of drug have benzene ring
Nosterius thanks for link i am reading now

[Fluorine]

Non-polar drugs will generally cross the blood brain barrier more readily. If a drug is too polar it will have difficulty or perhaps never even make it. Polarity is also issue when trying to make your way to the blood stream. The benzene ring of drugs is fairly nonpolar so it's a good basis to build up from.

Aside this if a drug is working to inhibit/activate a site it's a good idea to work up from what naturally interacts with it. Take for example the neurotransmitters dopamine, epinephrine, norepinephrine, melatonin, and serotonin. All of them contain a benzene ring as part of their parent structure. If you want to alter their transport, receptor, metabolization, etc. they are examples of what works/fits the site. This is where pi-pi interaction that Arkcon mentioned comes into play, though it is not the only possible interaction. Some drugs can exist without this but for many it's a strong foundation.

[fledarmus]

Also, benzene is a nice rigid system and there is a lot of chemistry available for generating almost any substituent pattern you like. This allows you to put functional groups into very specific rigid geometries, something that is very helpful in interacting with proteins.

[amintobello]

Thanks everyone for answers i wasn't know non-polarity makes easier to cross blood-brain barrier so it could be answer. I thought similarity with hormones but it wasn't the answer i was looking for because if we say similarity we have to ask why hormones have benzene  so non-polarity makes sense thanks again to everyone