Large amounts of alcohol make HIV worse. Alcohol can interact very badly with ARVs. Alcohol is also bad for your liver. Most medicines are broken down in the liver so you do not want anything to damage your liver. This is important if you are on ARVs. Lastly alcohol stimulates your libido (makes you want to have sex) and can cause you to lose control. Always be prepared to only have safe sex when you go drinking. Being drunk also diminishes your skill to use a condom.
Attribution: Straight Talk Foundation & Marie Stopes