The Journey of a Drug: What Happens Before Clinical Trials?
By Taylor Wilcox
9/30/2025
When we think about medicines, we often picture the little pills we pick up at the pharmacy or the injection a doctor prescribes. But how does a drug actually come to be? The path from an idea in a laboratory to a treatment tested in patients is long, expensive, and very carefully regulated. Before any drug ever enters a clinical trial, it goes through years of discovery and development.
What Exactly Is a Drug?
The U.S. federal government defines a drug as “any article intended for use in the diagnosis, cure, mitigation, treatment, or prevention of disease.” In simpler terms, a drug is something designed to help detect, treat, or prevent illness.
There are two main types of drugs:
Small molecule drugs – These are the traditional medicines most people are familiar with. They are chemically made by scientists in a lab (imagine a chemist mixing compounds in test tubes). Because of their small size, these drugs can often be taken by mouth in pill form, and they can pass through cell membranes to reach targets inside the cell.
Biologics – These are much larger, more complex drugs made from living systems rather than purely chemical reactions. Instead of being “cooked up” in a lab, biologics are created by inserting the gene for a desired protein into a cell or bacteria, which then produces the protein. Scientists can grow these cells in large fermentation tanks and purify the drug from there. Examples include insulin for diabetes and Humira® for autoimmune diseases like Crohn’s disease.
Both types have advantages and disadvantages:
Small molecules can be taken orally and rarely trigger immune reactions, but they may have more side effects and often need to be taken more frequently because they’re broken down quickly in the body.
Biologics are usually very specific to their target (meaning fewer off-target side effects) and often last longer in the body, but they cannot reach targets inside cells, must be given by injection, and sometimes trigger immune responses.
Step 1: The Discovery Phase
Drug discovery begins with an idea. Scientists first study the biology of a disease to figure out what might be going wrong at the molecular level. They then identify potential “targets” — proteins, receptors, or pathways that could be altered to improve health.
Once targets are identified, researchers start looking for compounds that can affect them. This may involve:
Designing new chemical compounds in a lab,
Screening thousands of molecules from chemical libraries, or
Testing proteins (biologics) that could bind to or block the target.
At this stage, the process is like searching for a needle in a haystack. For a single disease, scientists might begin with around 15,000 possible compounds.
Step 2: From Hits to Leads
Not all compounds are equal. Researchers run experiments, called assays, to see how well each candidate affects the disease target.
Hits are compounds that show some promise.
Leads are the most promising hits that get optimized — researchers tweak their structures or production process to make them more effective, safer, or longer lasting.
By the end of this stage, those initial 15,000 candidates may be narrowed down to a few hundred.
Step 3: Preclinical Testing
Before a drug can ever be tested in humans, it must go through years of preclinical research. This usually takes 5–6 years.
In this stage, the drug is studied extensively in:
Cell cultures (human or animal cells in a dish), and
Animal models (mice, rats, or other animals that can mimic aspects of the disease).
Researchers look for answers to questions like:
Does the drug actually reach the target?
Does it improve the disease process in animals?
How safe is it? (Are there toxic effects?)
How is it processed in the body (absorbed, distributed, metabolized, eliminated)?
Is there a good chance it will be effective in humans?
This stage is also when chemists refine the drug’s structure and when biologists optimize how a biologic is produced at scale. By the end of preclinical testing, the list of candidates is narrowed even further, usually down to about 200 or fewer.
Step 4: Filing an IND
Once a drug looks safe and effective enough in preclinical studies, researchers can apply to move it into human testing. To do this, they must file an Investigational New Drug (IND) application with the U.S. Food and Drug Administration (FDA).
An IND includes:
Results from all the laboratory and animal testing,
Evidence the drug is likely safe for humans,
Information on how the drug will be made and tested for quality,
A proposed drug label, and
Plans for clinical trials (including who will be eligible and how safety will be monitored).
The FDA reviews the application to decide whether the potential benefits outweigh the risks. Only after approval can the drug move into clinical trials in humans.
Why Does It Take So Long?
The entire process before a clinical trial can take close to a decade and cost billions of dollars. This is because safety is the top priority. Most drug candidates fail somewhere along the way — some don’t work as expected, some cause harmful side effects, and some are too difficult to manufacture.
But this rigorous process ensures that by the time a drug reaches a patient in a clinical trial, there is strong evidence it may work and that it is reasonably safe to test in humans.
The Takeaway
Drug discovery and development is like building a bridge between an idea and a treatment. It starts with identifying the right target, screening thousands of possible compounds, carefully testing them in the lab and in animals, and refining the drug into its safest, most effective version. Only after years of work and an FDA review does a drug ever reach the stage where it can be tested in people.
The next step in the journey — clinical trials — is where patients play a critical role in helping to determine whether a new treatment truly works.
