An antidote or counterdose is a substance which can counteract a form of poisoning. The term ultimately derives from the Greek αντιδιδοναι antididonai, "given against".

Sometimes, the antidote for a particular toxin is manufactured by injecting the toxin into an animal in small doses and the resulting antibodies are extracted from the animals' blood. The venom produced by some snakes, spiders, and other venomous animals is often treatable by the use of these antivenins, although a number do lack one, and a bite or sting from such an animal often results in death. Some animal venoms, especially those produced by arthropods (e.g. certain spiders, scorpions, bees, etc.) are only potentially lethal when they provoke allergic reactions and induce anaphylactic shock; as such, there is no "antidote" for these venoms because it is not a form of poisoning, though anaphylactic shock can be treated (e.g., by the use of epinephrine).

Some other toxins have no known antidote. For example, the poison ricin, which is produced from the waste byproduct of castor oil manufacture, has no antidote, and as a result is often fatal if it enters the human body in sufficient quantities.

Ingested poisons are frequently treated by the oral administration of activated charcoal, which absorbs the poison, and then it is flushed from the digestive tract, removing a large part of the toxin.

Poisons which are injected into the body (such as those from bites or stings from venomous animals) are usually treated by the use of a constriction band which limits the flow of lymph and/or blood to the area, thus slowing circulation of the poison around the body.

Poison and toxic signs

• Anticholinergic poisoning is given physostigmine sulfate as the antidote.
• atropine sulfate poisoning is given Anticholinesterase and pralidoxime chloride 2-PAM as the antidote.
• Benzodiazepine poisoning is given flumazenil as the antidote.
• Beta blocker poisoning is given glucagon as the antidote.
• Carbon monoxide poisoning is given oxygen as the antidote.
• Cyanide poisoning is given amyl nitrite, sodium nitrite, or thiosulfate as the antidote. A new patented process uses mesoxalic acid as the antidote.
• Digoxin poisoning is given Fragment antigen binding(Fab) fragments that bind to digoxin (trade names Digibind and Digifab) as the antidote.
• Ethylene glycol poisoning is given ethanol or fomepizole as the antidote.
• Extrapyramidal reactions associated with antipsychotic poisoning is given diphenhydramine hydrochloride and benztropine mesylate as the antidote.
• Heavy metal poisoning is given chelators, calcium disodium edetate (EDTA), dimercaprol (BAL), penicillamine, and 2,3-dimercaptosuccinic acid (DMSA, succimer) as the antidote.
• Heparin poisoning is given protamine sulfate as the antidote.
• Iron poisoning is given deferioxamine mesylate as the antidote.
• Isoniazid poisoning is given pyridoxine as the antidote.
• Methanol poisoning is given ethanol or fomepizole as the antidote.
• Methemoglobinemia poisoning is given methylene blue as the antidote.
• Opioid poisoning is given naloxone hydrochloride as the antidote.
• Paracetamol (acetaminophen) poisoning is given N-acetylcysteine as the antidote.
• Thallium poisoning is given Prussian blue as the antidote.
• Warfarin poisoning is given vitamin K phytomenadione and fresh frozen plasma as the antidote.