The toxins attach to proteins inside human nerve cells
Scientists are on their way to developing an effective antidote for botulinum toxin - one of the world's most feared biological weapons.
Defence experts say that just one gram of the poison can kill hundreds of thousands of people.
Several people each year fall victim to "botulism" from food poisoning, but the toxin is also used as Botox - injected into brows to relax wrinkles.
The US team's findings appear in the Journal of Biological Chemistry.
With funding from the US government, researchers at the Brookhaven National Laboratory, New York, and the United States Army Medical Research Institute of Infectious Diseases (USAMRIID), Maryland, have broken through a barrier towards developing an effective antidote against the most potent form of the toxin.
The researchers have developed a protein that blocks the effects of the toxin by tricking it into not attacking cells in the body.
Biologist Subramanyam Swaminathan, who led the research, told BBC News: "We anticipate at least four to five years before this can be turned into an approved drug."
The Clostridium botulinum bacterium produces seven different neurotoxins, which attach to proteins inside human nerve cells and blocks the chemicals they use to communicate with each another and with muscles. This can paralyse breathing muscles, which eventually suffocates the victim.
The new protein developed at the Brookhaven National Laboratory acts on the most powerful of these seven toxins, for which there is no medical treatment.
The team developed a protein that blocks the effects of the toxin
It behaves as a decoy to proteins in the nerve cells, which means that the toxin chooses not to attach itself to the nerve cells when it enters the body. This prevents paralysis.
"It is about 10 to 15 times better than the best one available so far," said Subramanyam Swaminathan.
Vaccines for botulinum toxin already exist, designed to be administered before an attack, but this research could produce a drug that would work afterwards.
The US government has proposed increasing funding for research into defence against bioweapons such as botulinum to $9bn (£4.5bn; 5.8bn euros) in 2009. This is a rise of more than 5% on the previous year.
Although botulinum toxin has never been successfully used as a bioweapon, the Japanese terrorist cult, Aum Shinrikyo, tried three times between 1990 and 1995.
Also, in the run-up to the 1991 Gulf War, Iraq reportedly produced thousands of litres of the toxin.