The use and scientific acceptance of “n-of-1” trials have accelerated in recent years, especially in connection with personalized medicine and drug discovery. This increase in interest and acceptance is reflected in the number of studies on the field.
A Google Scholar search (July 26, 2020) for “n-of-1 trial validity” for all date ranges, yielded 3,430 results. By contrast, the same search string for the period 2019-2020, yielded 527 results. That search found 221 results in the first seven months of 2020.
Those studies reflect a broad acceptance with major studies comparing n-of-1 trials to Randomized Control Studies and other long-accepted research methods [1,2,3,4,5]
This article from the British Medical Journal  notes:
“Well conducted, randomised clinical trials create the evidence base that informs modern medical practice, guidelines, and regulatory decisions. However, these trials estimate average treatment effects, providing little information about heterogeneity and no information about individual treatment effects.
“Randomised clinical trials are not always feasible, are rarely conducted on rare diseases, and often exclude individuals with comorbid conditions or concurrent treatments even though these individuals constitute the majority of patients.
“Based on the methodology of randomised clinical trials, n-of-1 trials can rigorously measure intervention effects for individuals, either singly or in a series.”
 SPIRIT – Standard Protocol Items: Recommendations for Interventional Trials
 CONSORT – Consolidated Standards of Reporting Trials