There is no guarantee. It depends on how old the lettuce is, meaning how long had it been in the grocery store before you bought it. It also depends on whether it has any bacteria like botulism or E. Coli or others. There are many things to be taken into consideration. In short, there are many factors that come into play. To play it safe, soak and wash your lettuce thoroughly, change the water every 1-3 days, and ensure it doesn't have any mucus on it before using it.