I'm 16. on 40ml Accutane, on my 4th month and I drank a beer last night and smoked cigarettes. Have I damaged my liver?

As a skin care expert I recommend a healthy diet free from the use of alcohol and other prescriptions that may cause injury to the  liver.  Honestly, a single episode of alcohol use will probably not cause any long term effects, however it will be best to maintain a clean, healthy diet free from alcohol and high cholesterol foods.  Smoking cigarettes is a poor choice for anyone for long term lung health.  As far as the lab test goes you should be honest with your physician and let them know of your poor choice moment so they can continue to give you the best of care.