Almost every clinical trial includes "concurrent participation in another interventional clinical trial" as an exclusion criterion. The word "interventional" is the key — it refers to trials where you receive an active treatment, drug, device, or procedure. Observational trials (where researchers only collect data without giving you anything) may be excluded less strictly.
The safety argument
When two investigational compounds are active in your system at the same time, neither research team can know with certainty whether an adverse event came from their drug, the other drug, or an interaction between the two. That's a problem for the data — but more immediately, it's a problem for your care team, who may not be able to correctly attribute or treat a side effect.
Investigational drugs don't have established interaction profiles with each other. Standard drugs have decades of interaction data; investigational compounds have months or years at best. Running two simultaneously creates an interaction unknown that no one can manage safely.
What is typically allowed
Observational studies — where you're providing data (blood samples, surveys, biomarkers) but not receiving any intervention — are usually compatible with treatment trial enrollment. These don't affect what's in your system and don't complicate the treatment trial's attribution of outcomes.
Registry studies and long-term follow-up studies from a previously completed trial are similarly usually fine. If you completed a trial last year and are still in an observational follow-up phase, that typically won't exclude you from a new interventional trial.
Sometimes the restriction is time-based rather than absolute. A trial might say "no participation in an interventional trial within the past 30 days" — meaning if you left your last trial a month ago, you're eligible. The washout window clears the previous compound from your system.
Always disclose
The practical takeaway: if you're in a trial and are contacted by another team about a different study, disclose your current participation immediately. The screening process will ask you about this. Concealing it is dangerous — it's your health at stake — and it's also a violation of the consent agreement you signed.
If you're genuinely interested in a second trial that seems highly relevant to your situation, ask both study teams about washout requirements, and involve your treating physician. The answer may be "wait and apply to the second trial after completing the first."