Clinical trials are simply put developmental experiments done (on humans or animals) to see the reaction of the drug under observation and gather a dataset of results for further analysis of the drug.
Positive results is what company and the market normally expects because that increases chances of drug approval (FDA drug approval) in some ways. In other words, company has to prove that drug is safe and effective to solve the defect for which it is manufactured.
For instance, if I were the CEO and I had to introduce a new drug for humans in the market, I want to make sure that the effect of the drug doesn't vary from one person to another. For that, I would do clinical trials first, by paying off volunteers to come in and try out the drug. And since every drug has its pre-requisites for clinical trials, not everyone can volunteer for these clinical trials. These volunteers should meet that criteria in order to participate in clinical trials. For example, if it is a drug for cancer, then patients with cancer would be best fit for clinical trials.
We see drug warnings on the box when we buy any drug; these I think are created during this R&D phase, which clinical trials are part of. For instance, if patient has cancer and cold how will drug react? All of such scenarios, I think, are carefully considered during clinical trials to get broadest data-set possible in order to cover all possibilities.
Hope this helps. I am not the doctor nor am I the CEO ready for clinical trials but at least that is what I think what clinical trials are.
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