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Heals the user by 1/2 of its maximum HP if no weather conditions are in effect, 2/3 of its maximum HP if the weather is sun, and 1/4 of its maximum HP if the weather is hail, rain, or sandstorm, all rounded half down.
The PP and reduced recovery in other weather conditions beyond sun hold this move back, but Synthesis is still usable. If your OU sun team seems to have little trouble keeping sunlight up, consider throwing Synthesis on any Pokemon that can spare the moveslot to keep them in good health. In RU and NU, where there are no automatic weather inducers, Synthesis is a good choice for keeping any bulky Grass-type Pokemon you may have, such as Eviolite Roselia and Tangrowth, in prime condition.
Recover is an equivalent that is unaffected by weather.
Slack Off is an equivalent that is unaffected by weather.
Milk Drink is an equivalent that is unaffected by weather.
Softboiled is an equivalent that is unaffected by weather.
Roost is a Flying-type equivalent that grounds the user and is unaffected by weather.
Heal Order is a Bug-type equivalent that is unaffected by weather.
Wish is an equivalent, but HP is recovered at the end of the second turn.