For most adult cats, clumping litter is safe. It can be riskier for kittens though. When they discover something new, they tend to explore it with their mouths and may swallow litter, which can block their digestive systems. Some adult cats do this too. In both cases, it's best to choose a non-clumping litter, such as silica-based litter, absorbent litter, or natural litter.
If you suspect your cat has swallowed litter and starts vomiting, we recommend seeing your veterinarian as soon as possible.