"It is a very foul tasting potion that must be taken for a week every night leading to the full moon. But it will lessen the effects of the curse. You will still transform, but the wolf is docile. You'll be able to sleep through the transformation, act more human if you are awake."

Remus wouldn't promise this if he didn't know they have an access to it now. Snape had promised to brew it. And there's Lily who also can provide the potion. Hogwarts is well stocked with ingredients.