Developing a new medicine, including a new antibiotic, takes many years and has several stages. The first stage is discovery. Scientists find or design a chemical that might treat a disease. It may come from plants or microorganisms, or be made in a laboratory. Penicillin was found in a mould.
The next stage is development, where the chemical is improved and made into a form that can be given as a medicine. Then comes preclinical testing. The drug is tested in the laboratory on cells and tissues, and on live animals, to check that it is safe and that it works.
If it passes, clinical testing begins in humans. At first the drug is given in a low dose to healthy volunteers to check that it is safe. Then it is tested on patients to find out whether it works and what dose is best. In a clinical trial one group may be given a placebo, which looks like the drug but has no active ingredient, so the effect of the drug can be compared.