An IPotionEffect is an IPotion that also has a duration and an amplifier (strength) parameter.
You can get them from an IPotion object.

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import.
import crafttweaker.potions.IPotionEffect;

ZenGetter	Type
ZenGetter duration	Type int
ZenGetter potion	Type IPotion
ZenGetter doesShowParticles	Type bool
ZenGetter amplifier	Type int
ZenGetter curativeItems	Type List<IItemStack>
ZenGetter effectName	Type string
ZenGetter isAmbient	Type bool
ZenGetter isPotionDurationMax (also a ZenSetter)	Type bool

Uses an IItemStack.
Returns a bool

ZenScript
 Copy
isCurativeItem(IItemStack item);

Uses an IEntity or any of its subclasses (e.g. IPlayer).
Returns void (nothing).

ZenScript
 Copy
performEffect(IEntity Entity);