Withdrawal of participants from randomized trials can occur because of symptoms thought to be related to the study medicine, but the causal relationship between the study medicine and the symptoms is often unclear. Single-patient trials (‘N-of-1 trials’), developed to identify optimal therapy for an individual patient in the clinical setting, may provide a means of resolving some of these dilemmas. We describe here the use of an N-of-1 study embedded within a placebo-controlled trial of saw palmetto for a participant who considered withdrawing because he believed the study medication caused an increase in his blood pressure. In this case, the N-of-1 study not only reassured the patient, who decided to remain in the study, but provided potentially useful new information regarding the study medication. Wider use of formal N-of-1 studies may be a valuable tool for improving adherence and determining whether observed side effects are caused by study medication in clinical trials.