What it does
Synthesis restores 1/2 of the user's maximum HP under neutral weather, 2/3 under harsh sunlight, and a punishing 1/4 under rain, Hail, or Sandstorm. Those three tiers are the entire strategic context of the move.
In Champions format there's no ambient weather — weather is always summoned by a Pokémon's ability or move, runs for a set number of turns, then clears. That means you can predict when Synthesis will be hitting the good number vs. the bad one. Running Synthesis into a sand or rain team and expecting 1/2 recovery is just wrong — you're getting 1/4, which is worse than most passive berries.
In doubles, the opportunity cost of using Synthesis is a full turn of not attacking or supporting your partner. That's a real cost. It only makes sense when the Pokémon using it genuinely has nothing else urgent to do that turn and staying alive matters more than tempo.
Who runs it
Florges (15% usage) is the cleaner use case. It runs support sets — Moonblast, Aromatherapy, and status-scrubbing duties. Synthesis fills the slot for turns where Florges isn't needed to heal a partner or control the field. Its extremely high Special Defense means it can survive an attack to use Synthesis the following turn.
Serperior (6% usage) uses Synthesis opportunistically. With 113 base Speed and Contrary as a primary mechanic, Synthesis on Serperior shows up mostly in dedicated sun-team contexts or slow-paced matchups where it can afford the recovery turn without losing position.
How to use it
Under neutral weather, Synthesis is roughly equivalent to a manually activated Sitrus Berry — except berries don't cost your action. The move is worth it when your partner is handling the immediate threat, giving you a free turn to heal.
Sun context is where Synthesis earns its slot. If your team has a sun setter — anything with Drought — Synthesis at 2/3 recovery under harsh sunlight effectively stretches your Florges or Serperior's staying power significantly. Attack one side with your sun sweeper, heal the support, repeat. The cycle holds for several turns as long as sun stays up.
Tips & strategy
The biggest danger with Synthesis is bringing it into weather you didn't plan for. Opponent lead with rain setters? Your Synthesis just dropped to 1/4 for the rest of the game. In metagames with heavy weather pressure, Wish or Leftovers gives more consistent recovery without the weather vulnerability.
Under Trick Room, slower support Pokémon actually gain a small Synthesis window — they act first in the reversed order and can heal before the opponent responds. Faster Pokémon in Trick Room that need Synthesis to survive are usually in the wrong matchup; switching your lead combination is probably the right call.