Paxil 40 mg is a Selective Serotonin Reuptake Inhibitor. SSRIs like Paxil 40 mg work by increasing the availability of serotonin in the brain. Serotonin is a neurotransmitter that plays a key role in regulating mood, anxiety, and emotional well-being. By inhibiting the reuptake of serotonin, Paxil helps increase serotonin levels in the brain, which can help improve mood, reduce anxiety, and alleviate other symptoms associated with mental health disorders.