Objective To analyse the frequency of adverse events to treatment reported by patients in relation to consultation time, attention from their usual doctor and information provided by their doctor about treatment.

Design Descriptive study. Patients were invited to respond to a telephone survey.

Setting 21 Primary Care health centres in Spain.

Participants 15 282 patients attended by GPs or paediatricians (error of 1% for p=q=0.50, α 95%) were selected at random from the total consultations recorded in 1 month. For sampling, quotas were assigned for type of attention, age and sex. In the case of children (under 14 years), the survey was answered by their parents.

Main outcome measures Patients' report on frequency of unexpected or adverse reaction to a treatment; whether informed or not about possible complications of the treatment and precautions to take; consultation time; and whether or not patient is usually seen by the same doctor.

Results 1557 (17.6%, CI 95% 16.8 to 18.4%) of the adults and 867 (13.7%, 95% CI 12.8 to 14.5%) of the children reported adverse or unexpected reactions to the treatment according to patients' reports. Consultation time (OR 0.5, 95% CI 0.4 to 0.5), doctor rotation at the health centre (OR 2.04, 95% CI 1.85 to 2.25) and information on treatment precautions (OR 0.47, 95% CI 0.43 to 0.53) determine the higher risk of adverse reactions to treatment.

Conclusions Planning at health centres should involve the monitoring of mean consultation time and doctor rotation as indirect indicators of safety. Furthermore, protocols related to the information provided to patients should be reviewed.