Clopidogrel to Prasugrel in Acute Coronary Syndrome (ACS) Patients

Inclusion Criteria: * Participants hospitalized with acute coronary syndrome (ACS) \[unstable angina (UA), non-ST elevation myocardial infarction (NSTEMI), or ST elevation myocardial infarction (STEMI)\] as determined by the investigator, and who are anticipated to undergo percutaneous coronary intervention (PCI) as a treatment for the ACS event within 24 hours of the clopidogrel/placebo loading dose * Participants provide signed informed consent form (ICF) * Participants weigh at least 60 kilograms (kg) at the time of screening * Women of child-bearing potential (that is, women who are not surgically or chemically sterilized and who are between menarche and 1-year postmenopause), test negative for pregnancy at the time of enrollment based on a urine or serum pregnancy test Exclusion Criteria: * Have cardiogenic shock at the time of randomization (systolic blood pressure greater than 90 millimeters of mercury (mm Hg) associated with clinical evidence of end-organ hypoperfusion, or participants requiring vasopressors to maintain systolic blood pressure over 90 mm Hg and associated with clinical evidence of end-organ hypoperfusion * Have refractory ventricular arrhythmias * Have New York Heart Association (NYHA) Class IV congestive heart failure * Have systolic blood pressure greater than 180 mm Hg, or diastolic blood pressure greater than 100 mm Hg on more than 1 assessment at any time from participant presentation of ACS treatment to enrollment * Have received fibrin-specific fibrinolytic therapy less than 24 hours prior to randomization * Have received nonfibrin-specific fibrinolytic therapy less than 48 hours prior to randomization * Have active internal bleeding or history of bleeding diathesis * Have clinical findings, in the judgment of the investigator, associated with an increased risk of bleeding * Prior history of ischemic or hemorrhagic stroke * Intracranial neoplasm, arteriovenous malformation, or aneurysm * Prior history of transient ischemic attack (TIA) * Have an International Normalized Ratio (INR) known to be greater than 1.5 at the time of evaluation * Have a platelet count of less than 100,000 per cubic millimeter (mm\^3) at the time of evaluation * Have anemia \[hemoglobin (Hgb) less than 10 grams per deciliter (g/dL)\] at the time of evaluation * Have received 1 or more doses of a thienopyridine (ticlopidine, clopidogrel, or prasugrel) or other adenosine diphosphate (ADP) receptor inhibitor within 10 days prior to screening * Have been administered glycoprotein IIb/IIIa (GPIIb/IIIa) inhibitor within the past 7 days or planned use of a GPIIb/IIIa inhibitor during PCI * Are receiving or will receive oral anticoagulation or other antiplatelet therapy, other than aspirin (ASA), which cannot be safely discontinued for the duration of the study. * Are receiving daily treatment with nonsteroidal anti-inflammatory drugs (NSAIDs) or cyclooxygenase-2 (COX2) inhibitors that cannot be discontinued during the study * Are currently enrolled in, or discontinued within the last 30 days from, a clinical trial involving an investigational drug or device or off-label use of a drug or device, or concurrently enrolled in any other type of medical research judged not to be scientifically or medically compatible with this study * Have previously completed or withdrawn from this study or any other study investigating prasugrel * Are women who are known to be pregnant, who have given birth within the past 90 days, or who are breastfeeding * Have a concomitant medical illness (for example, terminal malignancy) that in the opinion of the investigator, is associated with reduced survival over the expected treatment period * Have known severe hepatic dysfunction (that is, with cirrhosis or portal hypertension) * Have a history of intolerance or allergy to aspirin or approved thienopyridines (ticlopidine, clopidogrel or prasugrel) * May be unable to cooperate with protocol requirements and follow-up procedures