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Objectives To illustrate and quantitate how herding in oncology drug development (five or more biopharmaceutical companies competing to develop modulators of an identical target) amplifies the number of patients enrolled in clinical trials of experimental drugs, and to estimate the cost of trials.Methods and analysis In the setting of globally based clinical trials in oncology drug development, two public databases (ClinicalTrials.gov and International Clinical Trials Registry Platform), registering clinical trials, were interrogated to determine patient enrolment numbers and clinical trial details for the development of inhibitors of the immune checkpoint protein TIGIT (T-cell immunoreceptor with Ig and immunoreceptor tyrosine-based inhibitory motif domains). Estimates were made of the costs of trials to biopharmaceutical companies and of the numbers of anonymous patients enrolled in clinical trials.Results 21 companies are or were seeking to have 30 TIGIT inhibitors approved for clinical use in a case of herding. Nearly 49 000 patients were enrolled in 220 clinical trials of TIGIT inhibitors between May 2016 and September 2025 at an estimated cost of between US$3.1 billion and US$3.6 billion. Development of 8 of 30 (27%) TIGIT inhibitors in oncology has been terminated to date (November 2025) due to their failure to demonstrate significant clinical activity or for strategic reasons. Nearly 15 000 patients were enrolled in these terminated trials. None of the TIGIT inhibitors had received marketing authorisation as of November 2025.Conclusions Because of the high attrition rate (95%) in the development of drugs for oncology, herding amplifies the loss of human and financial resources incurred during drug development, with large number of patients enrolled in clinical trials of experimental drugs that are unlikely to receive approval for clinical use. Herding concentrates resources at the expense of investment in a broader portfolio of drug development programmes with a spread of risk against attrition.