Why most promising new drugs fail before they reach the chemist

Of every ten thousand chemical compounds that scientists hope might cure a disease, only one or two will ever make it onto a pharmacy shelf. The rest are weeded out by a clinical trial system designed to be a brutal, multi-stage obstacle course.

Many people assume that once a drug works in a laboratory, the rest of the process is just bureaucratic box-ticking. In reality, each stage of testing asks a completely different question. A drug that looks like a miracle cure in a petri dish can easily turn out to be useless—or dangerous—when it enters the human body.

What happens in phase one?

Once a drug has been tested on cells and animals, it enters Phase 1. This is the first time the compound is put into humans. The goal here is not to cure anyone; it is simply to find out if the drug is safe and how much of it a body can take.

These trials are small, usually involving between 20 and 100 healthy volunteers who are paid for their time. Doctors start with tiny doses and slowly increase them, monitoring the volunteers closely for side effects. If a drug causes severe liver damage or dangerous heart rate spikes even at low doses, the project is abandoned. At this stage, researchers also map how the body processes the drug—how it is absorbed, broken down, and excreted.

Why do drugs fail in phase two?

If a drug proves safe in healthy people, it moves to Phase 2. This is the first time the drug is tested on actual patients—usually a few hundred people who have the condition the drug is designed to treat.

This phase asks a fundamental question: does it actually work? Scientists are looking for what they call proof of concept. They also use Phase 2 to figure out the right dosage for sick patients, which can be very different from the dosage tolerated by healthy volunteers.

The failure rate at this stage is high, often hovering around 70 per cent. A drug might pass Phase 1 because it is perfectly harmless, only to fail Phase 2 because it simply has no effect on the disease. Human biology is incredibly complex, and a mechanism that successfully killed cancer cells in a laboratory mouse often fails to do the same in a living human being.

What makes phase three the ultimate test?

Phase 3 is the most expensive and time-consuming part of the process. It requires thousands of patients, often spread across multiple hospitals and countries.

To pass, the drug must prove not just that it works, but that it works better than what is already available, or at least has fewer side effects. These trials are randomised and double-blind. Neither the patients nor their doctors know who is getting the new drug and who is getting a placebo or the existing standard treatment.

Many drugs that sailed through Phase 2 fail spectacularly here. In a small Phase 2 trial of 50 highly selected patients, a drug might look like a triumph. But when tested on 5,000 diverse patients—who might be older, have other health conditions, or be taking other medicines—the treatment's benefits can wash out. Rare, dangerous side effects that only affect one in a thousand people might also only show up once you test thousands of patients.

What happens after a drug is approved?

If Phase 3 is a success, the manufacturer submits the data to regulators, such as the Medicines and Healthcare products Regulatory Agency (MHRA) in the UK.

But the testing does not stop when the drug is approved. Phase 4, or post-marketing surveillance, goes on indefinitely. Regulators and drug companies monitor how the drug performs in millions of patients in the real world. If a very rare side effect—such as a one-in-a-million blood-clotting issue—emerges once the drug is widely used, regulators can change the warning labels, restrict who can take it, or pull it from the market entirely.