AIMS: Clinical trial patients are highly motivated but may encounter difficulty in taking study medication regularly when treatment burden is substantial. We assessed a brief behavioural intervention, given in addition to a standard trial protocol. METHODS: We performed a two-arm adherence sub-study within a twelve-month randomised controlled drug trial evaluating the impact of statin and/or omega-3 EE90 treatment in 800 patients with type 2 diabetes. Fifty-nine United Kingdom general practices were cluster-randomised to action-planning or control groups. The former delivered an initial written exercise prompting participants to formulate action-plans to take study medication regularly, with brief nurse encouragement to use action-plans at later visits, whilst the latter followed the standard trial protocol. The primary outcome was proportion of days on which study medication were taken as intended measured by electronic medication containers. RESULTS: Adjusted mean (95% CI) proportion of days with medication taken as intended was 79.3% (76.3-82.3%)for the 30 action-planning practices (321 participants), compared with 78.5% (75.8-81.1%) for 27 control group practices (426 participants, with a mean intervention effect of 0.9%, 95% CI -3.1% to +4.9%, p=0.67). Adjusted odds ratios for ⩾80% trial medication adherence for action-planning compared with control practices were 1.29 (0.90-1.84) and 1.38 (0.96-1.99) respectively. CONCLUSIONS: Low-intensity action-planning interventions used alone are unlikely to have a clinically important impact on medication adherence, particularly in a clinical trial setting. These findings, do not exclude their contribution, as part of a multifactorial intervention, to improving treatment adherence. ISRCTN number 76737502.