BACKGROUND: Outcomes of tailoring statin-type based on solute carrier organic anion transporterfamily member 1B1 ( SLCO1B1)pharmacogenetic toxicity information on patient, provider, and pharmacological outcomes are unknown. METHODS: The trial randomized 159 patients not taking statins because of prior statin myalgia 1:1 to receiving SLCO1B1 GIST (Genotype Informed Statin Therapy) versus usual care (UC) and followed for up to 8 months. The UC arm received their SLCO1B1 results post-trial. The primary outcome was statin adherence using the Morisky Medication Adherence Scale, which was assessed in those patients who reinitiated statins. Secondary outcomes assessed in all participants included statin reinitiation and LDLc (low-density lipoprotein cholesterol), within and post-trial. Using commercial laboratory data, serial LDLc were compared between 1907 patients receiving SLCO1B1 testing and propensity-matched, untested controls. RESULTS: Trial participants were 25% SLCO1B1*5 carriers. Statin adherence was similar between arms (Morisky Medication Adherence Scale in GIST versus UC, 6.8±1.5 versus 6.9±1.6, P=0.96). GIST led to more new statin prescriptions (55.4% versus 38.0%, P=0.04) and lower LDLc at 3 months (131.9±42.0 versus 144.4±43.0 mg/dL; P=0.048) with similar magnitude at 8 months (128.6±37.9 versus 141.0±44.4; P=0.12). SLCO1B1*5 carriers exhibited a greater drop in LDLc with GIST versus UC (interaction P=0.048). Post-trial, LDLc decreased in UC participants who crossed over to GIST compared with those allocated to GIST (-14.9±37.8 versus +9.0±37.3 mg/dL, P=0.03). Patients tested for SLCO1B1 though a commercial laboratory had a greater LDLc decrease ( P=0.04) compared with controls. CONCLUSIONS: Delivery of SLCO1B1 pharmacogenetic testing that addresses statin myalgia improved statin reinitiation and LDLc but did not improve self-reported statin adherence. CLINICAL TRIAL REGISTRATION: URL: https://www.clinicaltrials.gov . Unique identifier: NCT01894230.