inhibitor

One who, or that which, inhibits.

I found daily meditation to be a useful inhibitor of negative thoughts.

Any substance capable of stopping or slowing a specific chemical reaction.

The class of antidepressants most commonly associated with sexual dysfunction is selective serotonin reuptake inhibitors, or SSRIs, which treat depression by ultimately increasing levels of the neurotransmitter serotonin in the brain. […] A common solution is switching to bupropion, which is a norepinephrine-dopamine reuptake inhibitor, or NDRI.

Any substance capable of stopping or slowing a specific biological process.