Before antidepressants are allowed to be marketed by government regulatory agencies, they have to pass stringent tests proving not only that they work, but that they are safe also. This is why it costs literally hundreds of millions of dollars to develop, research and produce a new antidepressant agent.
When a company approaches any of the innumerable governments of the world to purchase their antidepressant for population usage, government experts try hard to find reasons to disprove the claim. When a medication is bought by multiple countries, it confirms this medication has been proven to be effective.
Antidepressants are tested in a strict scientific manner against antidepressants we already know to be effective, and against placebo (inactive agents made to look like the antidepressant being tested). The statistical techniques used ensure that the benefits of an antidepressant cannot have arisen by chance. Using these mathematical techniques, any antidepressant has to show that it is dramatically more effective than placebo, and must also show that it is as effective as antidepressants we already know to be effective. Many antidepressant drugs have failed these tests, so that only the most clearly effective agents are allowed on the market.
With regard to the percentages who do respond to various agents, human nature is such that 30% of people will respond to taking a placebo! Some of these people are simply improving by coincidence with the passage of time, while others are responding to the idea they are actually having active treatment, even though the substance itself is inactive. In fact, many of these placebo responses are only temporary. In contrast, 70% of patients will respond to any one of the proven antidepressants on the market, and their response will be long lasting usually. (In fact, it does appear that one third of people responding to an antidepressant get incomplete remission of their symptoms, or become resistant to the antidepressant, so the antidepressant has to be changed.)
To be accepted on the market, with government approval, antidepressants have to show by probability statistics that their superior efficiency compared to placebo is at least 95% certain, by probability statistics, not to be a chance finding. While claims about effectiveness in depression are made for many non-medical substances, scientific medicine and psychiatry insists that any of the substances must be able to prove its effectiveness in placebo controlled double blind studies described above, ie where neither the patient nor the prescribing doctor in the trial knows until after the end of the trial whether the patient has been taking placebo or the agent being tested.
If your doctor prescribes you an antidepressant, you will receive a substance which has been extensively tested and has faced rigorous examination to ensure its chances of working are high. Unfortunately, we do not yet have a way of predicting which antidepressant will work for which individual person, so that it may sometimes be necessary to try more than one antidepressant agent to find one which suits your chemistry. Doctors normally choose an antidepressant for you on the basis that they are all equally effective, but they all have different side-effects.
Important Disclaimer: This site is medical information only, and is not to be taken as diagnosis, advice or treatment, which can only be decided by your own doctor.