There are several problems with human testing of psychiatric medications. This first set of issues is none generally incurred in all testing of new drugs, namely that if trials work, it seems unfair to deprive control groups of the drugs, but if the drug is dangerous, it is not ethical to give potentially harmful substances to test subjects. Another issue is that the profit motive often leads drug companies to skew the data by eliminating unresponsive test subjects.
There are difficulties special to testing psychiatric medications. First, given that many episodes of schizophrenia resolve without any interventions, it may be unethical to treat people with medications which may have potentially adverse side effects. Another problem is that degrees of mental illness are very difficult to measure effectively, and thus it is hard to make provable claims about the validity of tests.
Also, as many of these drugs are addictive or cause physical harm, there are moral dilemmas revolving around their use and testing.