Ive seen it countless times in british crime mysteries. Not sure about american frothing.
So, does this happen when you are poisoned? Cyanid for example? And if so, why?
Or is it movie licence?
Need answer fast?
Frothing at the mouth (due to excess salivation mixing with air) can be due to a variety of causes including drug ODs and poisons, sometimes reflecting pulmonary edema which also can be due to overdoses and poisoning with such things as cyanide.
I’ll leave it to you to do specific searches, but remember, computer forensic experts can find them… 
*frothing at the mouth could also be due to rabies. Never overlook rabies.
It’s mostly a movie trope, as is instantaneous death.
Cyanide results in very quick death, either from ingestion or inhalation, which is why it was used in gas chambers. I don’t think it leads to frothing at the mouth however.
Different poisons certainly react in different ways and at different speeds. Arsenical poisoning, for example, can resemble gastritis, and can produce death over a long period of smaller doses, or in a few hours of a larger dose.
Agatha Christie was an expert at poisons, due to her career in WWI as a chemist’s (pharmacist’s) technician, added to further research once she started writing. Some writers, including screenwriters, care about accuracy, others not so much. There are lots of fictional depictions of poisonings that are basically correct.
Minutes. Not seconds like you see in many films.
Notably averted in Madame Bovary.
Also, some intentionally screw up the details, just so that their fictional work can’t be used as a “how to” in real life.
Screenwriters also have to be concerned about actual screen time and pacing. Poisons act far too quickly. Choking someone to death also takes a LOT longer in real life. TV and movie people don’t say even say goodbye on the phone, just to save a few seconds of screen time. Relationships often develop far too quickly.
What is realistic and what is entertaining to watch during a limited time period are two very different things.
As the saying has it: “Movies are real life with all the boring parts edited out.”
Including that boring 15 minutes between ingesting the e.g. cyanide and dying from it.
Tell me about it. I wish someone had warned me.
Uh, I mean, it’s something I heard about. Once.
I haven’t seen “frothing” used so much since the last time I was in Starbucks!
I like “spuming” better.
(Somebody fix the title, it’s driving me nuts)
I might expect frothing from poisoning via acetylcholinesterase blockers, such as nerve agents or organophosphate pesticides. They produce (among other things) copious salivation. It’s one class of poison for which I can think of a reason for frothing at the mouth.
As Jackmannii said, a substance that produces pulmonary edema is another reason for the mouth frothing.
For most poisons I can’t think of any mechanism for the movieland style of frothing.
Apparently much faster in the form of gas, such as a few seconds.
Tetrodotoxin, i.e. the Fugu poison from pufferfish and also many other aquatic or semi-aquatic critters, can cause hypersalivation. This can lead to foaming. But it kills in hours, not seconds.
I had a dog taste a toad once. He frothed.
Could be a hold over from when people had rabies, which is very uncommon now. And a rabid person, dog, or any animal will foam at the mouth because they can’t swallow or drink anything, hence the other name, hydrophobia.
A dog will froth at eating a crawdad.
Dogs seem to froth alot.
My Siamese cat froths when I make him take his monthly pill. But he does it to piss me off.
24 hours, unless The Simpsons lied to us.
Well…I hate to disillusion you about our toppest-tier media, but…
The victim, although completely paralysed, may be conscious and in some cases completely understandable until shortly before death, which generally occurs within 4 to 6 hours (range ~20 minutes to ~8 hours).
via wikipedia
Actually, ingested cyanide can lead to frothing of the mouth:
A 43-year-old healthy male, who is a goldsmith by occupation and works in a jewelry factory was brought to the emergency department (ED) by his wife with an alleged history of ingestion of white crystals (unknown compound) of an unknown quantity. His wife noticed two episodes of vomiting immediately following ingestion. He was brought to the ED approximately 15 minutes after the ingestion of the compound. At the presentation, the patient was frothing from the corners of the mouth and was gasping with tachycardia. He was immediately intubated and mechanically ventilated.
Another account of cyanide poisoning and frothing: