Here's why:
* Some varieties are safe: Romaine lettuce, butter lettuce, and red leaf lettuce are generally considered safe.
* Some varieties are less safe: Iceberg lettuce, and other varieties with high water content and low nutritional value, can cause digestive upset.
* Limited quantities: Lettuce should only make up a small portion of a guinea pig's diet.
* Variety is key: Guinea pigs need a diverse diet of hay, fresh vegetables, and pellets.
Important Note: It's always a good idea to consult with a veterinarian for personalized advice on your guinea pig's diet.