# Why clinical trials stop

Sample: **8,000 terminated interventional studies** from ClinicalTrials.gov, 30 October 2026 (capped at 8 pages).
Registry denominators: 34,631 terminated, 16,941 withdrawn, 333,500 completed, 65,221 recruiting.

| Reason category | n | share |
|---|---:|---:|
| other/unspecified | 3081 | 38.5% |
| enrollment | 2391 | 29.9% |
| efficacy | 1008 | 12.6% |
| funding | 502 | 6.3% |
| business | 422 | 5.3% |
| safety | 411 | 5.1% |
| covid | 185 | 2.3% |

## By phase (top reasons)

- **EARLY_PHASE1** (n=105): other/unspecified 37%, enrollment 28%, efficacy 13%
- **NA** (n=3242): other/unspecified 42%, enrollment 33%, efficacy 9%
- **PHASE1** (n=922): other/unspecified 41%, enrollment 17%, business 13%
- **PHASE1/PHASE2** (n=426): other/unspecified 34%, enrollment 25%, efficacy 14%
- **PHASE2** (n=1732): enrollment 33%, other/unspecified 32%, efficacy 16%
- **PHASE2/PHASE3** (n=151): other/unspecified 40%, efficacy 19%, enrollment 19%
- **PHASE3** (n=819): other/unspecified 40%, efficacy 22%, enrollment 22%
- **PHASE4** (n=603): enrollment 41%, other/unspecified 38%, efficacy 11%

## By sponsor class

- **FED** (n=59): other/unspecified 44%, enrollment 20%, efficacy 14%
- **INDIV** (n=12): efficacy 50%, other/unspecified 25%, enrollment 25%
- **INDUSTRY** (n=2746): other/unspecified 39%, enrollment 17%, efficacy 15%
- **NETWORK** (n=97): enrollment 44%, other/unspecified 38%, efficacy 7%
- **NIH** (n=231): other/unspecified 55%, enrollment 24%, business 10%
- **OTHER** (n=4760): enrollment 37%, other/unspecified 37%, efficacy 12%
- **OTHER_GOV** (n=94): other/unspecified 45%, enrollment 34%, efficacy 9%
- **UNKNOWN** (n=1): safety 100%

## Method note

`whyStopped` is free text, so reasons are assigned by keyword regex into six categories plus 'other'.
Categories can overlap in the source text; the first match wins, in the order efficacy, safety,
enrollment, funding, business, covid. This is a coarse but reproducible classification.

## Limits

- 8-page cap: the sample is terminated studies in registry order, not a random sample.
- Free-text reasons are inconsistently written; 'other/unspecified' is large.
- Withdrawn studies are excluded here (separate status), so pre-start cancellations are missed.
- Registration practice varies by sponsor and country.