Special Serum is a rare item that, when merged with another item, will immediately upgrade that item to its next level!

For example:

1 x Level 11 Watering Can + 1 x Level 4 Serum = 1 x Level 12 Wheelbarrow

Make sure your Special Serum is fully merged to Level 4 before use.

Special Serum cannot be used on standard Producers, but can be used on your Event Producers.

As a special item, it's best used wisely on the most challenging merge chains.