8 obstacles to success in human trials

Human trials are how we develop and validate evidence for therapeutics, dietary interventions, devices, and lifestyle approaches. A well-formed hypothesis and a solid protocol matter enormously, but they do not guarantee success. Knowing where trials most commonly run into trouble is part of designing one that does not.

1. Participant recruitment

Finding enough eligible participants within the available timeframe is consistently one of the hardest parts of any trial. Tight inclusion and exclusion criteria, limited public awareness of the study, and geographical constraints all reduce the pool.

The scale of the drop-off can be stark even in a well-run study. A monitored enrollment study for a Veterans Affairs and NIH acute kidney injury trial screened 4,339 patients, of whom only 2,744 met inclusion criteria, and after further exclusions only 1,710 remained potentially eligible. Of those, just 65.7% actually enrolled. Impediments to informed consent alone excluded over a fifth of potentially eligible patients. The same study found that simply modifying an overly restrictive enrollment window cut the loss of eligible patients from 6.6% down to 2.3%, a reminder that some recruitment leaks are fixable protocol design problems, not immovable facts about the population. The question worth asking early is whether the expected effect size is realistic for the participant population you can actually access, and whether your funnel from screened to enrolled has any similarly fixable narrow points.

2. Funding

Trials cost more than they look like they will. Beyond the obvious expenses (personnel, equipment, study products), there are hidden costs in software, specialist expertise, and the time required for compliance and quality work. Budget planning needs to account for the full picture, including the possibility of replication if your hypothesis requires it for robust validation.

3. Regulatory clearance

High-risk interventions require regulatory authority clearance before any participant can be enrolled. The FDA, EMA, and MHRA each have their own timelines and requirements. This process can be lengthy, and delays here push back everything that follows. Lower-risk studies may not need the same level of clearance, but understanding your regulatory position before you plan your timeline is essential.

4. Participant retention

Getting people into a study is one challenge; keeping them is another. Participants withdraw for many reasons: adverse events, loss of interest, changed personal circumstances, or simply finding the study more burdensome than anticipated. High dropout rates reduce statistical power and can introduce bias if the people who leave are different in some relevant way from those who stay. Minimising burden and maintaining engagement from the start is easier than recovering from high attrition.

5. Data management

Poor data management does not just create administrative problems: it undermines the validity of everything you collect. Inconsistent capture methods across sites, unsecured storage, and inadequate audit trails all create problems that may not become visible until the study is over. Validated, secure data systems are not an optional upgrade; they are foundational to producing results you can stand behind.

6. Site management

Multi-site trials introduce coordination complexity. Maintaining consistent procedures and data collection standards across geographically distributed sites requires active oversight. Language barriers add another layer when staff or participants span different regions. The more sites involved, the more explicit and detailed your site management approach needs to be.

7. Lack of participant interest

Even when your recruitment strategy is sound, generating genuine interest in participation is a separate challenge. People may be unaware of the study, uncertain about its safety, or put off by a protocol that seems demanding. Studies that are perceived as burdensome struggle to recruit and retain. Simplicity in participation design is a competitive advantage.

8. Ethical standards

Ethical violations do not just create legal exposure; they invalidate the work. Ethics review bodies exist to protect participants, and studies that fail to meet their standards face rejection, reputational damage, and in serious cases, legal consequences. Ethical compliance is not a one-time hurdle at the start of a study: it is an ongoing commitment throughout.

Where the fix actually lives

Several of these obstacles look like fixed facts about the world (not enough eligible people, not enough budget) when they're often partly a design problem in disguise, as the enrollment window example above shows.

The eight obstacles above are all manageable with proper planning. The ones that tend to derail studies are the ones that were not taken seriously early enough, and as the enrollment research above shows, sometimes the fix is smaller and more mechanical than the obstacle first appears.