**Core Concept**
Niacin toxicity, also known as niacin overdose, occurs when excessive amounts of nicotinic acid are ingested, leading to adverse effects on various bodily systems. The primary concern with niacin toxicity is its impact on the cardiovascular and gastrointestinal systems.

**Why the Correct Answer is Right**
Niacin toxicity is primarily mediated through its effects on the liver, where it is metabolized into nicotinuric acid, which can cause liver damage and increase liver enzymes. Additionally, niacin toxicity can lead to flushing, itching, and gastrointestinal symptoms such as nausea, vomiting, and diarrhea. This is due to its vasodilatory effects and its ability to inhibit prostaglandin synthesis, leading to increased prostaglandin levels in the gut.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect as niacin toxicity can indeed cause liver damage and increase liver enzymes, making it a plausible concern in cases of niacin overdose.

**Option B:**
This option is incorrect as niacin toxicity is not typically associated with hypoglycemia; rather, it can cause hyperglycemia due to its effects on glucose metabolism.

**Option C:**
This option is incorrect as niacin toxicity can indeed cause gastrointestinal symptoms such as nausea, vomiting, and diarrhea, making it a plausible concern in cases of niacin overdose.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that niacin toxicity can be mitigated by administering aspirin or ibuprofen, which can reduce the severity of flushing and gastrointestinal symptoms by inhibiting prostaglandin synthesis.

**Correct Answer: D.**