Strange Brew

Two separate rolls happen when you drink one, and they are not tied to each other. How much health you get back is random, and how long you are locked out afterward is random, so a bad drink can hand you the smallest heal and the longest wait at the same time. Averaged out it edges past a Healing Potion, but an average is no comfort in the two seconds where you needed a full heal and got the low roll.

Most drinks also grant a brief patch of invincibility, and that is the thing that actually separates this from ordinary healing. Timed against an incoming attack it can swallow the hit outright, which nothing else available before hardmode does. Short bursts are far more common than long ones, so treat it as a bonus you sometimes get rather than a defensive tool you can rely on.

The Skeleton Merchant only stocks it on certain moon phases, and he wanders the caverns rather than living in your town, so you cannot plan around him. Buy what you find, keep ordinary potions as your real supply, and shorten the lockout with a Philosopher's Stone or a Charm of Myths if you want this to be more than a curiosity.