You are not alone, many alli consumers have asked this question before. You may do a quick search in this forum to find previous discussions.
If you want to "treat yourself to a fatty meal" you will need to stop the pill 2 to 3 days before, especially if you have a slow digestive system. Read below to know how long alli remains in your system:
The half life of the drug is 1-2 hours. Since everyone has different GI transit times, I think the drug can actually remain in your system for up to 48-72 hours mainly because gastric emptying time is so variable from person to person.
In the other hand, I will encourage you to treat yourself with something less fattening, specially while you are trying to lose weight. How about a massage, a pedicure or manicure, a new outfit, a vacation, a new hair cut, etc. I think you will feel much better :)
Hope this helps,
Sylvia Melendez-Klinger, MS, RD, LD, CPT - registered dietitian