BACKGROUND: Informed consent is the foundation of the ethical conduct of health research. Obtaining informed consent may unwittingly interfere with the data collected in research studies, particularly if they concern sensitive behaviours that participants are requested to report on. To address gaps in evidence on such research participation effects, we conducted a methodological experiment evaluating the impact of the informed consent procedure on participants' reporting behaviour, specifically on their self-report of drinking behaviour as measured by Alcohol Use Disorder Identification Test (AUDIT). METHODS: A two arm double blinded randomised controlled trial was used. University students present in London student unions at the time of recruitment were contacted in two phases (an initial run-in phase followed by the main phase). Those providing positive responses to verbal questions: 1) "are you a student?"; 2) "do you drink alcohol?"; 3) "would you like to take part in a brief health survey, which will take around 5 minutes?" were recruited. Participants received one of the two envelopes by chance, with the sequence generated by an online random sequence generator. One contained the participant information sheet, informed consent form and the AUDIT questionnaire (the intervention group), while the other contained only the AUDIT questionnaire (the comparator group). The primary outcome was the mean AUDIT score, which ranges from 0 to 40. The secondary outcome was the proportion of participants in each group scoring 8 or more on the AUDIT, the threshold score for hazardous and harmful drinking warranting intervention. RESULTS: A total of 380 participants were successfully recruited, resulting in 190 participants in each group, of which 378 were included in the final analysis. There is no evidence of any statistically significant difference between groups in the primary outcome. A statistically significant difference in the secondary outcome was found in the run-in phase only, and not in the main phase, or overall. Moreover, between-group outcome differences between the two phases suggest an important influence of setting on reporting behaviour. CONCLUSIONS: There is no strong evidence that completion of informed consent itself alters self-reporting behaviour with regards to alcohol, though the effect of settings needs to be further studied.