Paracetamol is probably the most straightforward answer to your question. A dose of about 15-20 tablets of 500 mg (7.5-10 grams) is enough to cause severe liver failure, with a dose exceeding 30-40 tablets (15-20 grams) significantly increasing the likelihood of death without medical intervention. The toxicity comes from the production of a metabolite called NAPQI, which destroys liver cells. Symptoms begin within 24 hours with nausea, vomiting, and abdominal pain, while liver damage becomes evident within 24-72 hours, leading to multi-organ failure and potential death by the third or fourth day. While it's one of the most lethal over-the-counter drugs, medical intervention with N-acetylcysteine within 8-24 hours can reverse the damage, so it's not 100% guaranteed. Additionally, factors like liver health and alcohol consumption can affect the lethal dose.