**Core Concept**
Selective Serotonin Reuptake Inhibitors (SSRIs) primarily act by inhibiting serotonin reuptake in the synaptic cleft, enhancing serotonergic transmission. Common adverse effects include gastrointestinal disturbances, sexual side effects, and weight changes, but galactorrhea is not a typical or well-documented effect of SSRIs.

**Why the Correct Answer is Right**
SSRIs can cause weight changes (often gain or loss), gastrointestinal symptoms like diarrhea, and sexual side effects such as delayed ejaculation due to serotonergic effects on the central nervous system and genital pathways. However, galactorrhea—prolactin-induced milk production—is not a recognized adverse effect of SSRIs. While some SSRIs may mildly increase prolactin levels, this effect is minimal and not clinically significant enough to cause galactorrhea. The condition is more commonly associated with dopamine receptor blockade (e.g., in antipsychotics) or pituitary disorders.

**Why Each Wrong Option is Incorrect**
Option A: Weight changes are a well-documented side effect of SSRIs, often due to altered appetite regulation and serotonin-mediated effects on metabolism.
Option B: Diarrhea is a common gastrointestinal side effect, possibly due to serotonin's role in motility regulation in the gut.
Option C: Delayed ejaculation is a known sexual side effect of SSRIs, resulting from serotonergic inhibition of ejaculation pathways in the spinal cord.

**Clinical Pearl / High-Yield Fact**
SSRIs commonly cause weight changes, diarrhea, and sexual dysfunction, but galactorrhea is not a typical adverse effect—this makes it the best choice for a "not an adverse effect" question. Always remember: **galactorrhea is linked to dopamine antagonism, not SSRIs**.

✓ Correct Answer: D. Galactorrhea