In the earliest games of D&D poison has often served as just an instant save or die. The issue with it is twofold:

1. Poison doesn't work like that. Most poisons take a while to take effect and cause various things other than just instantly falling over and dying.
2. It isn't very fun. Once a character gets exposed to a poison they either survive or die, regardless of player's skill/actions.

Therefore, I propose a revised system of handling poison: When a character receives a dose of poison, it doesn't take effect during its latency period. This can be anything from a few seconds (as in the case of cyanide) up to even a few days (like with polonium). Afterwards, its effects show up, strengthening over a certain period until the Turning Point. Only then a save is made - if it succeeds, the person escapes the effects of the poison, without any permanent damage or at least alive, depending on the poison. If they fail - they either die or are permanently damaged by it. The save is rolled with advantage if the poisoned person avoided activity/caused vomiting/took antivenom or anything similiar. It can also be rolled with disadvantage due to a high dosage or activity spreading the poison.

Example Poison

Method of administration: Oral, intravenious, muscle injection, inhalation or contact. Latency Period: Time before any signs show (except for trouble breathing if the poison is inhaled or tingling if it is contact) Effects: From the earliest to the latest. All the things that do not depend on if you feel the whole effects of the poison. Turning point: Time before the poisoned person rolls a save to see if they feel the full effects of the poison Effects of Success: The lesser effects of the poison and how long they last. Effects of failure: The awful fate that meets a person who took the poison. Usually permanent, regardless if it is death or long-term health consequences. Sometimes a person can be saved even afterwards, but that requires a miracle.

Beneath are some examples of poisons made using these rules(note - they are not medical advice):

Cyanide

Method of administration: Oral.
Latency Period: 10 seconds.
Effects: shortness of breath, dizziness, convulsions \ Turning point: 1 minute. \ Effects of Success: Vomiting, the effects go away after d4+1 days. \ Effects of failure: Death in 2d4 minutes.

Arsenic (Acute)

Method of administration: Oral, Injection.
Latency Period: 2d4 hours.
Effects: Headache, severe diarrhea, vomiting, pain in the abdomen, nausea, convulsions, metalic taste
Turning point: 2d4 hours
Effects of Success: The acute symptoms go away after a week, but long term issues remain. Roll d4: 1 = partial paralysis in hands and feet, 2 = seizures, 3 = night blindness, 4 = intestinal cancer. \ Effects of failure: multiple organ failure kills the victim in d4 hours of painful convulsions and vomiting. Even if the organs do not fail, then the loss of fluids does the job.

Atropine (Aka Belladonna, taken from Deadly Nightshade, although it may also come from potatoes)

Method of administration: Oral, injection, bodily membranes (eyes, nose, anus). \ Latency Period: (2d4)10 minutes. \ Effects:* fever, flushing, blurring of vision, relaxation of muscles, hallucinations and delirium. \ Turning point: 12 hours \ Effects of Success: A coma for d4* (dice explodes) days, then you get better. \ Effects of Failure: A coma for d4* (dice explodes) days, then you die.

Snakebite (minor, aka most of the vipers and smaller rattlesnakes.)

Method of administration: Injection. \ Latency Period: d4 minutes. \ Effects: Immediate pain and swelling of the bitten part, fever, lowered pressure, internal bleeding. \ Turning point: 6 hours. \ Effects of Success: The bitten part of the body gets better after around a week. \ Effects of Failure: Gangrene sets in and the bitten part of the body has to be amputated.

Snakebite (major, aka the cobras and other big bastards)

Method of administration: Injection. \ Latency Period: 1 minute. \ Effects: Blistering around the wound, laboured breathing, rapidly lowering pressure, vomiting and diarhea, increased salivation and sweating, blurred vision, muscle twitching. \ Turning point: d610 minutes \ Effects of Success:* Permanent neurological issue to the bitten limb (-2 to all tests that use it, if it was a leg then a limp). \ Effects of Failure: Quick death due to your heart stopping and your lungs ceasing to work.

Nerve Gas (I'm using Sarin as an example, but most are quite similiar)

Method of administration: Inhalation, skin contact \ Latency Period: d6* minutes (exploding) \ Effects: muscle twitching in exposed areas, vomiting, nausea, inability to relax muscles, fatigue, trouble breathing, incontinence, paralysis \ Turning point: d65 minutes \ Effects of Success:* The effects remain for up to a few days, but muscle weakness, partial paralysis and chronic pain remain. \ Effects of Failure: Death due to inability to breathe, or choking on your own vomit or your heart stopping.