According to the Dietary Guidelines for Americans, 2010, moderate drinking is [1-p.31]:
For men: 2 drinks per day or 14 drinks per week
For women and adults after 65: 1 drink per day or 7 drinks per week
Possible Health Benefits of Moderate Drinking
Reduced risk of coronary heart disease, heart attack and heart failure due to increased blood levels of HDL and decreased levels of LDL cholesterol and triglycerides, and decreased blood coagulation and thus clot formation [1-p.31; 3,4]. Drinking up to two drinks per day does not affect the blood pressure . It is still not clear if consuming red wine is more protective to the heart than consuming other alcohol beverages .
Reduced risk of the hardening of the arteries (atherosclerosis) in the limbs, but only in non-smokers [1,2]. Alcohol dilates arteries, but not enough to significantly increase the blood flow through the already narrowed atherosclerotic arteries in the limbs .
Reduced risk of obesity , diabetes type 2 [1,2]chronic atrophic gastritis caused by H. pylori (in individuals who drink less than four drinks per week) , gallstones [1,2], ischemic stroke (caused by a clogged brain artery) [1,2], osteoporosis in women after menopause , cognitive decline in old people and death 
Reduced risk of kidney cancer, lymphoma and thyroid cancer . Long-term wine drinking may somewhat protect against esophageal, stomach and colorectal cancer . The protective effect of polyphenols quercetin, rutin and resveratrol in wine is uncertain, though.
The sensitivity for the beneficial effects of moderated alcohol drinking is partly genetically determined .
Possible Harms of Moderate Drinking
Increased risk of breast cancer . According to one estimation, each gram of alcohol per day increases the risk of breast cancer by 1%, possibly due to increased estrogen levels . Taking 400-600 mg of folate per day might reduce the risk of breast cancer in moderately drinking women [1,2].
Progression to heavy drinking, especially in the persons with family history of alcoholism 
Increased risk of nerve damage (peripheral neuropathy) in diabetics 
Increase of the blood triglyceride levels in those who already have hypertriglyceridemia 
Increased risk of reactivation of the inflammatory bowel disease (Crohn’s disease, ulcerative colitis) [12,13]
Heavy or High-Risk Drinking
Heavy or high risk drinking means :
For men: 5 drinks or more at a single occasion or 15 drinks or more per week
For women and adults after 65: 4 drinks or more at a single occasion or eight drinks or more per week. Women and adults after 65 tend to have less body water than men before 65, so they may achieve certain blood alcohol concentrations after fewer drinks than men before 65.
According to the National Institute on Alcohol Abuse and Alcoholism in the U.S., binge drinking is drinking that brings the blood alcohol concentration (BAC) to 0.08% (g/100 ml blood) or above , which may result, for example, from 3 drinks in an hour by a 140 lbs (65 kg) woman or 4 drinks in an hour by a 160 lbs (73 kg) man .
Dangers of Binge Drinking
Increased risk of irregular heart rhythm (arrhythmia). Heart rhythm disturbances after the weekend drinking are known as “holiday heart syndrome”.
Sudden increase of blood pressure 
Chest pain (in individuals with heart-related chest pain) due to a coronary artery spasm several hours after consumption (often in the morning) 
Increased risk of sudden cardiac death 
Hypoglycemia or alcoholic ketoacidosis, especially in individuals who have not eaten for few days, or in diabetics taking insulin or glucose-lowering drugs [11,19]
Adolescent who engage in binge drinking are at increased risk to become alcoholics .
Binge alcohol drinking inhibits the synthesis of proteins and may thus reduce the muscle-growing (anabolic) effect of protein intake and exercise .