Too much medicine is an increasingly recognised problem, and one manifestation is inappropriate polypharmacy in older people. Polypharmacy is usually defined as taking more than five regular prescribed medicines. It can be appropriate (when potential benefits outweigh potential harms) but increases the risk of older people experiencing adverse drug reactions, impaired physical and cognitive function, and hospital admission. There is limited evidence to inform polypharmacy in older people, especially those with multimorbidity, cognitive impairment, or frailty. Systematic reviews of medication withdrawal trials (deprescribing) show that reducing specific classes of medicines may decrease adverse events and improve quality of life. Two recent reviews of the literature on deprescribing stressed the importance of patient involvement and shared decision making. Patients and clinicians typically overestimate the benefits of treatments and underestimate their harms. When they engage in shared decision making they become better informed about potential outcomes and as a result patients tend to choose more conservative options (eg, fewer medicines), facilitating deprescribing. However, shared decision making in this context is not easy, and there is little guidance on how to do it. We draw together evidence from the psychology, communication, and decision making literature (see appendix on thebmj.com). For each step of the shared decision making process we describe the unique tasks required for deprescribing decisions; identify challenges for older adults, their companions, and clinicians (figure); give practical advice on how challenges may be overcome; highlight where more work is needed; and identify priorities for future research (table).