Stamina will not regenerate if it is over 99. However, If a player has a stamina level that is greater than 0, they will get 99 stamina no matter what. For example, if a player has 2 stamina when they use the potion, they will exceed the limit and reach 101 stamina.
If no Stamina Potions are in the player's Inventory, it is possible to use an Orb in its place. This, however, is ill-advised, because orbs are much more valuable than 99 stamina.