Overview

Generally speaking the only difference between a medicine or drug used therapeutically or recreationally and a poison or toxin is the dose.

A medicine WILL be toxic and dangerous if the dose is too high or taken to frequently, likewise many toxins, venoms and poisons from animals, plants and minerals can be and ARE used therapeutical in medicine.

Much of our existing contemporary medicine originated in nature as naturally occurring chemicals, toxins, venoms or poisons that have been extracted, isolated, purified and sometimes structurally modified to produce new drugs, which are then prepared in to pharmaceutical preparations such as tablets, injections, suspensions, eye drops, creams, ointments etc., use to treat disease and illness.

With this in mind, pretty much any naturally occurring chemical, toxins, venoms or poisons from plants and animals such as spiders, scorpions, snakes, sea anemones, fish, molluscs (sea snails, cone shell) and other venomous creatures have the potential to be used as either a poison or medicine under the right circumstances.

Side note #1 (feel free to overlook): Since obviously, that’s an oversimplification and nothing is as straight forward as that!

The way in which a chemical, toxin, venom or poison may kill you, is also the way in which it can be used therapeutically. Think of it this way, the only reason why you should be taking medicine, because something is not working correctly, so you counteracting that illness with a substance that causes the opposite (normally dangerous) effect.
Other factors which are important in reality include genetic variations in metabolic enzymes, receptor or ligand polymorphisms, existing medical conditions and medications, specific conditions such as liver or kidney disease, diet and/or deficiencies, age, gender, weight, muscle mass, allergies or tolerance, route of administration, pharmaceutical formulation just to name a few.

Simplified Classification

Side note #2: The two following factors are why we have side effects.

Each TYPE has multiple SUBTYPES that can be locally active in multiple parts of the body: this is why they can cause symptoms in seemingly unrelated ways.
Furthermore it is not only possible, but highly likely that a single chemical, toxin, venom or poison can also act in MULTIPLE ways: this is due to structural similarities that exist between different types.

As a general rule, plant and animal-based chemicals, toxins, poisons and venoms can be divided in to the following SIMPLIFIED categories (not an exhaustive list).

Type 1: Sodium channel blockers: Can be used as a local anaesthetic, pain killer, and treatment for heart palpitations/arrhythmias and seizures/epilepsy
Type 2: Calcium channel blockers: pain relief, blood pressure, heart rate/rhythm, seizures/epilepsy, Alzheimer’s disease/other dementias and Parkinson’s disease.
Type 3: Potassium channel blockers and immunosuppressant: Can be used to treat autoimmune and inflammatory diseases such as gastrointestinal diseases (crohn’s disease, ulcerative colitis), CNS diseases (multiple sclerosis, dementia), liver (hepatitis), kidneys (nephritis), whole body/systemic disease (Lupus erythematosus), joint disease (rheumatoid arthritis), skin (psoriasis, dermatitis) and respiratory diseases such as asthma.
Type 4: Alpha receptor and Beta receptor blockers: Used to reduce blood pressure, slow heart rate, reduce force of heart contraction. They can also be used to treat prostate enlargement and eye conditions such as glaucoma.
Type 5: Alpha receptor and Beta receptor activators: These are your typical life-saving drugs used to "kick start" the heart when it has stopped or has slowed to dangerous levels, when blood pressure is dangerous low, to counteract life threatening allergic reactions (anaphylaxis), medical resuscitation and life support for patients with sepsis/blood infections/poisoning, keeping someone alive after a traumatic injury resulting in dangerous amounts of blood loss (will also require "replacement" blood or fluid), plus open then airways and stimulate breathing in respiratory illness such as asthma, which can also be life threatening condition.
Type 6: Neurochemical blockers and enhancers: Depending on what they are blocking or enhancing, they may either treat or worsen conditions such as pain, Parkinson’s disease, Alzheimer’s disease/dementia, seizures/epilepsy, depression, mania, hallucinations, and other psychological conditions such as schizophrenia.
Type 7: Platelet and Clotting factor inhibitors: Depending on whether the target is platelets or one of many clotting factors, they can be used to prevent blood clots that may cause, heart attacks, strokes and other clots/thrombi such as DVT (deep vein thrombosis).