There isn't a single perfect synonym for "multicellular" because it describes a very specific concept. However, depending on the context, you could use some of these alternatives:

General alternatives:

* Many-celled

* Complex

* Organized

* Plurisomatic (less common, more technical)

More specific alternatives, depending on context:

* Metazoan (specifically for animals)

* Eukaryotic (for organisms with complex cells, including multicellular organisms)

* Multicorporeal (meaning "having multiple bodies," but can be used metaphorically)

Ultimately, the best alternative depends on the specific context and what you want to emphasize.