Some poisons have an antidote, but this is the case in a surprisingly few instances. More likely, treatment will focus around preventing absorption by the stomach of whatever has been ingested.
Depending on how soon after ingestion treatment is received, it may consist of:
Induced vomiting (rarely used if the if the subject is unconscious).
Activated charcoal, which is a form of carbon that has been processed to make it extremely porous, meaning it has a very large surface area available to absorb the drugs that have been ingested, thus preventing their absorption by the stomach. Anyone not 100% certain that they want to achieve death may want to consider having some of this to hand as it is available over-the-counter (dose for poisonings is said to be 1 gram per kg of body weight).
Stomach pumping, where a tube is run through the nose into the stomach through which small amounts of warm water or saline are administered into the stomach and via a siphoning action removed again along with the contents of the stomach.
Forced diuresis, typically achieved by drugs or intravenous infusion. This pushes the kidneys to pass more urine (and hence the drug ingested).