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Multiwalled carbon nanotubes (MWCNTs) and pharmaceutical compounds are classified by the US Environmental Protection Agency as contaminants of emerging concern, with significant research devoted to determining their potential environmental and toxicological effects. Multiwalled carbon nanotubes are known to have a high adsorptive capacity for organic contaminants, leading to potential uses in water remediation; however, there is concern that co-exposure with MWCNTs may alter the bioavailability of organic compounds. Existing studies investigating MWCNT/organic contaminant co-exposures have shown conflicting results, and no study to date has examined the combined effects of MWCNTs and a common pharmaceutical. In the present study, juvenile fathead minnows (Pimephales promelas) were exposed to sublethal concentrations of the over-the-counter antihistamine diphenhydramine (DPH) in the presence of natural sediment for 10 d, with some treatment groups receiving MWCNTs. Addition of MWCNTs did not have a protective effect on DPH-related growth inhibition, and did not reduce the whole-body burden of DPH in exposed fish. Mass-balance calculations indicated that significant amounts of DPH were adsorbed to MWCNTs, and DPH concentrations in water and sediment were commensurately reduced. Bioconcentration factor and biota-sediment accumulation factor increased in the presence of MWCNTs, indicating that P. promelas accumulates DPH adsorbed to MWCNTs in sediment, likely by co-ingestion of MWCNTs during feeding from the sediment surface. Environ Toxicol Chem 2017;36:320-328. (c) 2016 SETAC