Polypharmacy is common among patients with heart failure with reduced ejection fraction (HFrEF). However, its impact on the use of optimal guideline-directed medical therapy (GDMT) is not well established. This study sought to evaluate the association between polypharmacy and odds of receiving optimal GDMT over time among patients with HFrEF. The authors conducted a post hoc analysis of the GUIDE-IT (Guiding Evidence-Based Therapy Using Biomarker Intensified Treatment) trial. Polypharmacy was defined as receiving ≥5 medications (excluding HFrEF GDMT) at baseline. The outcome of interest was optimal triple therapy GDMT (concurrent administration of a renin-angiotensin-aldosterone blocker and beta-blocker at 50% of the target dose and a mineralocorticoid receptor antagonist at any dose) achieved over the 12-month follow-up. Multivariable adjusted mixed-effect logistic regression models with multiplicative interaction terms (time × polypharmacy) were constructed to evaluate how polypharmacy at baseline modified the odds of achieving optimal GDMT on follow-up. The study included 891 participants with HFrEF. The median number of non-GDMT medications at baseline was 4 (IQR: 3-6), with 414 (46.5%) prescribed ≥5 and identified as being on polypharmacy. The proportion of participants who achieved optimal GDMT at the end of the 12-month follow-up was lower with vs without polypharmacy at baseline (15% vs 19%, respectively). In adjusted mixed models, the odds of achieving optimal GDMT over time were modified by baseline polypharmacy status (P-interaction < 0.001). Patients without polypharmacy at baseline had increased odds of achieving GDMT (odds ratio [OR]: 1.16 [95% CI: 1.12-1.21] per 1-month increase; P < 0.001) but not patients with polypharmacy (OR: 1.01 [95% CI: 0.96-1.06)] per 1-month increase). Patients with HFrEF who are on non-GDMT polypharmacy have lower odds of achieving optimal GDMT on follow-up.