The People potion is an elixir that can change anyone who drinks it to look human. It apparently doesn't alter the person's genetics, just their appearance. Venus drank one so that she could marry a human king, but her daughter Vina was born a slime anyways.
One of Mink's primary goals throughout the series is to find one so that she can look human and marry Dick Saucer (who is a dragon slayer). She eventually gets one but uses it on her friend Rufa who had been turned from an elf into a toad by Azetodeth.