Efficacy: What It Really Means When a Drug 'Works'

When you read that a drug is "90% effective," what does that actually mean? Efficacy is the proof a medicine has in controlled settings — usually clinical trials. But that number can hide a lot. This page shows how efficacy is measured, what changes it in real life, and how you can read claims without getting confused.

How efficacy is measured

Clinical trials compare a drug to a placebo or another treatment under strict conditions. Researchers pick a clear endpoint — for example, fewer asthma attacks or lower blood pressure — and measure the difference. Trials report results as absolute risk reduction, relative risk reduction, or numbers needed to treat. Relative numbers sound bigger. Always check the absolute numbers: they tell you the actual benefit for patients.

Trials control factors like dose, timing, and patient selection. That makes the results cleaner, but it also makes them less like everyday use. A trial might enroll younger, healthier people or exclude those on multiple drugs. That affects how useful the efficacy number is for someone in the real world.

What changes a medicine's results outside trials

Real-world effectiveness often falls short of trial efficacy. Why? People miss doses, mix drugs, or have other health issues. Side effects can make people stop a medicine early. Drug interactions — like atenolol with some OTC meds — can blunt benefits. Resistance is another big factor for antibiotics and antivirals: bacteria or viruses change over time, lowering efficacy.

Formulation and delivery matter too. An inhaler that works well in a study might be less effective if a patient uses it wrong. Devices, storage, and even cost affect whether someone sticks with a treatment long enough to get the benefit.

Population differences matter. A medicine tested mainly in one group may perform differently in another group. That’s why doctors use both trial data and real-world experience when choosing treatments.

So how do you judge claims? First, look for study type: randomized controlled trials are strongest. Check sample size and follow-up time. Look for peer-reviewed publications or trusted summaries. Watch for relative vs absolute claims. Read about side effects and dropout rates. If possible, find real-world studies or registry data that show how the drug performs outside labs.

This tag collects articles that help you read efficacy claims and apply them — from asthma inhalers and statins to antibiotics and weight-loss drugs. You’ll find breakdowns of trial results, plain-language comparisons of alternatives, and tips for talking with your doctor about what a drug will likely do for you.

Want practical next steps? Ask your clinician how trial populations compare to you, how side effects could affect daily life, and whether there’s long-term real-world data. If cost or access matters, look at alternatives and savings options. A confident decision comes from understanding both the headline efficacy and the real-world picture behind it.