Lettuce is a fun treat for rabbits. However, if you feed them too much your rabbit may get diarrhea. This happens when things you feed your rabbit aren't digested and absorbed before they reach the end of the digestive tract where bacteria start to ferment it.
Under normal conditions bacteria work hard to slowly digest the tough fibers found in hay. Their work produces high quality nutrients for your bunny but this can go wrong if you feed it too much easily fermented foods like lettuce that are like candy for bad bacteria. A meal full of easily fermentable fibers found in lettuce are quickly fermented by bad bacteria and their growth can cause a change in the pH of the rabbits digestive tract. This spells trouble and often leads to bloating and diarrhea.
Rabbits should eat a diet that is based upon pellets properly formulated according to their age. For all rabbits this means that their diet should be based primarily upon hay. Adult pet rabbits are healthiest eating a grain-free and soy-free pellet that is balanced with the natural vitamins and chelated minerals. The nutritional needs of baby rabbits are better met with a high-fat diet (like mamma's milk) that is high in fiber and low in starch. Feeding bunnies anything other than their scientifically balanced diet will simply dilute the nutrients they are getting from their pelleted diet and will alter the balance of protein and energy that they need to grow and stay healthy.
So, can rabbits eat lettuce? Yes they can but only as a treat.