In Nigeria, access to malaria diagnostics may be expanded if drug retailers were allowed to administer malaria rapid diagnostic tests (RDTs). A 2012 pilot intervention showed that short message service (SMS) reminder messages could boost treatment adherence to RDT results by 10–14% points. This study aimed to replicate the SMS intervention in a different population, and additionally test the effect of an expanded message about anticipated RDT access policy change on customers’ acceptability for drug retailers’ administration of RDTs. One day after being tested with an RDT, participants who purchased malaria treatment from drug shops were randomized to receive (1) a basic SMS reminder repeating the RDT result and appropriate treatment actions, (2) an expanded SMS reminder additionally saying that the ‘government might allow pharmacists/chemists to do RDTs’ or (3) no SMS reminders (i.e. control). Using regression analysis, we estimate intent-to-treat (ITT) and treatment effects on the treated for 686 study participants. Results corroborate previous findings that a basic SMS reminder increased treatment adherence [odds ratio (OR) = 1.53, 95% CI 0.96–2.44] and decreased use of unnecessary anti-malarials for RDT-negative adults [OR = 0.63, 95% CI 0.39–1.00]. The expanded SMS also increased adherence for adults [OR = 1.42, 95% CI 0.97–2.07], but the effects for sick children differed—the basic SMS did not have any measurable impact on treatment adherence [OR = 0.87, 95% CI 0.24–3.09] or use of unnecessary anti-malarials [OR = 1.27, 95% CI 0.32–1.93], and the expanded SMS actually led to poorer treatment adherence [OR = 0.26, 95% CI 0.10–0.66] and increased use of unnecessary anti-malarials [OR = 4.67, 95% CI 1.76–12.43]. Further, the targeted but neutral message in the expanded SMS lowered acceptance for drug retailers' administration of RDTs [OR = 0.55, 95% CI 0.10–2.93], counter to what we hypothesized. Future SMS interventions should show consistent positive results across populations and be attuned to message length and content before initiating a larger messaging campaign.