Ferromin's effects are limited. After boosting an adventurer's HP by a maximum of 200HP using the medicine, Ferromin will stop working, and any additional Ferromin used on the same adventurer will 'add' 0 HP. For example, if Tielle at Level 99 with 274 HP (Who's never drank a bottle in her life) went to an all you can drink Ferromin Buffet one day, she would be able to increase her Max HP to 474HP, exactly +200HP more. Unfortunately, she'd be really sick of Ferromin at that point, so any bottles she drinks will contribute absolutely nothing to her maximum HP from then on.
Ferromin's item details only say the minimal amount it will increase maximum HP. There is a random chance you can receive a point or more of Max HP with more expensive ones offering more variance to the bonus.
Floors with the "Consumables are now half as effective" and "Consumables are now twice as effective" news flash do not alter the effect of Ferromin.
Ferromin's "correct" translated name is actually supposed to be Fuelmin and can be seen advertised on the side of the SuperVend SDXK56 vending machine.