Stopping a Clinical Trial Early

Yes, it does happen.

A Clinical Trial can be stopped early for 4 reasons:

We will deal with each one in more detail


It is rare for a trial to be stopped early for efficacy. But it does happen. This means the study hypothesis is unexpectedly proven early within predesignated criteria. Continuing to expose subjects in the inferior arm to additional potential risks or keeping them from benefitting from the therapies in the superior arm is hard to justify ethically.

Due to the rarity when you see it happen, pay attention! The share price will usually skyrocket when it occurs.

It is important to note that stopping a trial early for efficacy will only occur with drugs who’s mechanism of action is known.  If it is a new class of drug, the trial will almost always continue to the end. This ensures the FDA, or EMA, have as much data as possible to thorougly anazlyze all aspects of the drug.


This is one of the most common reasons for stopping a trial. The risks to human subjects unexpectedly outweigh the benefits because of unexpected severe adverse events.

It is important to recognize how safety is defined here. Basically the bad events are outweighing the potential good.  This means that the worse a condition is, the more adverse events that will be tolerated.


The study hypothesis is shown to be unprovable within the constraints of the trial based on a statistical analysis of early study data, usually done at a planned interim analysis. In such cases, there is no benefit to balance against subjecting subjects to potential risks. When this occurs it is deemed unethical to continue the trial.


This is very rare, but it does occur. This can mean that funding vanishes, researchers cannot access eligible patients or study interventions, or other studies make the research question irrelevant.