The United States Food and Drug Administration (FDA) has introduced manufacturing standards for dietary supplements, including red yeast rice, to assure their identity, purity, strength, and composition. One supplement commonly used to self-treat high cholesterol, red yeast rice, may contain monacolin K, an ingredient identical to prescription lovastatin. We examined whether FDA’s manufacturing standards led to standard concentrations of the statin monacolin K in red yeast rice supplements. We analyzed 28 brands of red yeast rice supplements by ultra-high performance liquid chromatography-diode array detector-quadrupole time-of-flight mass spectrometry for monacolin K content. Monacolin K was not detected in two brands. In the 26 brands that contained monacolin K, the quantity ranged more than 60-fold from 0.09 to 5.48 mg per 1200 mg of red yeast rice. Following the manufacturers’ recommendations for daily servings, the quantity of monacolin K consumed per day would range more than 120-fold from 0.09 to 10.94 mg. Despite FDA manufacturing standards, strength and composition of red yeast rice supplements sold at mainstream retail stores in the United States remains unpredictable.