Minecraft: 1.12.2
Forge: 14.23.5.2847

Mods: 2
cyberware-1.12.2-0.2.11.20
jei_1.12.2-4.15.0.279

This is a guess on what this item does even though the tooltip says that it boosts tolerance by +25 but makes you less resistance to diseases & poison.

I didn't know what the "less resistance to diseases & poison" part meant but a quick look at the source says that it extends the timer for the Poison & Hunger effects. There's also something related to the "Neuropozyne" but unsure if that is affected. Anyways... I did some testing and noticed a few things.

Test: Effects from mobs.
The Poison Effect increases from '7 seconds to 12 seconds' if you got attacked by a Cave Spider, however that's only if the effect is applied for the first time, meaning that the effect will not refresh back to '12 seconds' but rather back to '7 seconds' if you kept taking damage from the Cave Spider after the boosted timer runs down.

The Hunger Effect increases from '7 seconds' to '12 seconds' if you got attacked by a Husk, however that's only if the effect is applied for the first time, meaning that the effect will not refresh back to '12 seconds' but rather back to '7 seconds' if you kept taking damage from the Husk after the boosted timer runs down.

Test: Effects from Potions & Food
Unsure if this is intended but the Suppressant only seems to increase the timer depending on how you get the Effect.

For the Hunger Effect, which you can get by eating certain foods like Raw Beef, Rotten Flesh, etc. The default timer is '30 seconds' via Rotten Flesh. The Suppressant doesn't appear to increase the timer.

For the Poison Effect... which you can get via Poison Potions or by eating a Pufferfish/Poisonous Potato, etc. The default timer for the Poison Potato is '5 seconds' while the Pufferfish timer is '1 minute' and for the Poison I Potion... the timer is either '45 seconds' or '1 minute & 30 seconds' while Poison II Potion is '27 seconds'. The Suppressant will not increase these timers however if you get hit by a Splash Potion version of it... it's timer will be increased, for example... if I got hit by a Splash Potion of Poison with a timer of '1 minute & 30 seconds' and it's a direct hit... the timer will go up to '2 minutes & 22 seconds' but only when the effect first applies which means any other Splash Potions of Poison will only refresh it back to the default time after the boosted timer runs down.

It seems that the boost only applies if you get the effects from Mobs or via Splash Potions so unsure if it's suppose to be like that.

Thanks for the detailed report.
Neuropozyne is an alternative to power consumption, it doesn't affect the Poison & Hunger potions.

Existing version only check for potion effect presence. As such, it shouldn't be affected by the source (potion drinking, food eating, damage effect, etc.). If I wait for the effect to completely expire, I can see food eating effects with an increased duration, as expected.

Existing version doesn't detect when a potion effect duration gets extended. This can only be fixed when duration is below the original value. That part will be fixed in next version.