It is important to think about the way in which this text presents us with the various characters roaming around aimlessly in Europe and following their decadent lifestyles. The role of alcohol in this is particularly important as it is an example of the dissolute lifestyle of the various characters as they waste their lives in hedonistic pleasures. However, the one thing that I would take issue with is that alcohol is certainly not the only hedonistic pleasure that the characters indulge in. Brett for example also embarks on a series of affairs that act as a form of compensation as well. Alcohol abuse is certainly one symptom, but it very definitely is not the only one. A thesis statement you could use therefore might be:
Alcohol and its abuse is one of the characteristcs of the dissolute lifestyles of the characters in The Sun Also Rises.
This would allow you to explore the purpose of drinking alcohol and how it fills a void in the lives of the characters in this novel.