Quick Answer: A Randomized Controlled Trial (RCT) is the only way to prove a drug actually works. By randomly assigning patients to receive either the real drug or a placebo, researchers ensure both groups are mathematically identical. Any difference in health outcomes can then be securely attributed to the drug, not a confounding factor.
The Danger of Observational Studies
Before RCTs, doctors used observational studies. If people who drank red wine lived longer, they declared red wine healthy. The flaw? Wealthy people are more likely to buy red wine, and wealthy people have better access to healthcare. The wine didn't make them live longer; the money did. This hidden factor is called a Confounder.
How Randomization Kills Confounders
When you take 10,000 people and RANDOMLY split them into two groups, the laws of probability ensure that both groups have roughly the same number of wealthy people, poor people, smokers, vegans, and people with hidden genetics. Because the groups are identical at the start, if the treatment group lives longer, the drug MUST be the cause.
The Double-Blind Requirement
Randomization isn't enough; humans are easily swayed by belief. In a double-blind trial, neither the patient nor the doctor knows who got the real pill and who got a sugar pill. This prevents the Placebo Effect in the patient, and prevents experimental bias in the doctor (who might unconsciously treat the "drug" group better).