The investigators examine the influence of esomeprazole versus famotidine on antiplatelet action of clopidogrel associated with aspirin. At least 100 consecutive patients suffering from acute coronary syndrome or undergoing coronary artery stent implantation , who received aspirin (80 - 160 mg/day) and clopidogrel (300 mg loading dose, followed by 75 mg/day or 75mg/day for at least 7 consecutive days), are randomised to receive either esomeprazole 20 mg daily vs famotidine 40 mg daily in a double blinded manner. Clopidogrel effect was tested by measuring residual platelet reactivity (RPR) to ADP by VerifyNow P2Y12 assay (Accumetrics Inc, San Diego, Calif). At baseline, whole blood will be obtained for RPR at least 12 h after clopidogrel loading dose or at least 7 days of maintaince dose. Immediately obtaining the baseline blood, patients will be randomized to receive either esomeprazole (20 mg/day) or famotidine 40 mg/day for 28 days. Double blinding will be performed by encapsulation of study drugs. RPR will be measured again at the 28th day.

The investigators will compare the % inhibition and the P2Y12 reaction Units (PRU) at the 28-day treatment period in the 2 groups.

Inclusion Criteria:

• patients suffering from acute coronary syndrome or undergoing coronary artery stent implantation (PCI) , who received aspirin (80 - 160 mg/day) and clopidogrel (300 mg loading dose, followed by 75 mg/day), are recruited.

Exclusion Criteria:

• known active peptic ulcer disease or gastrointestinal within 8 wk
• known iron deficiency anemia with Hb < 10 gm/dl
• mechanical ventilation
• active cancer, liver cirrhosis, end-stage renal failure
• life expectancy < 1 yr
• known allergic to aspirin, clopidogrel, famotidine or esomeprazole
• pregnancy, lactation, child-bearing potential in the absence of contraception,
• co-prescription of NSAID, corticosteroid, or warfarin
• non-oral feeding or impaired GI absorption e.g. vomiting
• patient on proton pump inhibitor within 4 weeks