Within the biomedical paradigm, treatment effects are typically split into primary and secondary effects with temporality playing a key role in this separation. Yet, this kind of ordering of effects with some effects understood as happening on the ‘side’, secondary and temporary, does not fit with how they are experienced by many patients who undergo treatment for cancer. Drawing on empirical data from a research project that gathered narratives of women's experiences of radiotherapy for gynaecological cancer, we observe radiotherapeutic effects that are experienced as ambivalent and temporally diverse and as overlapping demands that the women endure and manage. We propose Derrida's concept of pharmakon as a relevant and useful analytic for understanding radiotherapy treatment, thus bringing into focus the ambivalent effects of radiotherapy - it is both therapeutic and toxic. Pharmakon, we argue, offers a way of disrupting the logics that govern current practices of therapeutic radiotherapy, and provides a way to re-negotiate the ordering and temporal understandings and practices of therapeutic efficacy, outcome and accountability of radiotherapy treatment - away from a temporal fragmentation of treatment effects and patients' bodily experiences to a focus on how best to support the whole patient in living with the ambivalent, temporally diverse and overlapping effects and demands of treatment.