In Western medicine, there was a time when a famous doctor would proclaim the usefulness of a certain medicine and many of us in the medical profession would agree wholeheartedly, without questioning. Now, you need hard evidence. In the 1950s, it was Bradford Hill who started the process of doing proper drug trials to determine the efficacy of drugs.

The evidence-based concept is now called randomised controlled trial ( RCT). In the classic RCT, to determine if drug x works for a disease y, first you need an adequate sample size of patients with disease y who will be administered the drug x. To a similar number of patients you need to administer a sugar pill or a dummy pill, called a "placebo" in medical speak. Then over a certain period of time, if you can show that those in the drug arm of the trial did better than in the placebo arm of the trial, you may be able to conclude that the drug X is effective against the disease y. But the difference in effectiveness has to be what is called, "significant".

And this significance is determined by statistical testing which tries to eliminate "chance" as the cause for drug x being more effective. To eliminate the chance factor, randomisation is very important.

For example in determining who receives the placebo versus the drug in question, the choice has to be random. If the patient is pre-selected to receive the drug or the placebo then you have introduced bias (chance) into the study.
One of the first diseases to be subjected to human trials was tuberculosis. At one time only one drug was used for TB, then RCTs revealed that the disease responded significantly better to combination of drugs than just one drug. This lesson was carried over for a more modern scourge, the HIV virus against which a combination therapy is now utilized.

RCTs are now also made use of for studying the efficacy of vaccines. The injectable typhoid vaccine that is used worldwide was first studied here in Kathmandu by Dr I L Acharya and colleagues in the early 1980s using the RCT concept.

Finally, besides efficacy, adverse events or side effects of drug also have to be assessed properly to avoid the kind of tragedy that the drug "thalidomide" caused. So, a world of regulations has spawned around RCTs to make sure drugs are safe and effective.