Medicins Sans Frontieres says new stocks of Fav-Afrique, which neutralises 10 different snakebites that can occur in Sub-Saharan Africa, are desperately needed.
The last batch will expire in June 2016 and there is no comparable replacement.
Manufacturer Sanofi Pasteur says it has been priced out of the market.
Alternatives are available but MSF says they are not as good.
Fav-Afrique is the only anti-venom that has been proven safe and effective to treat envenoming from different types of snakes across Sub-Saharan Africa, it says.
Sanofi said it will share the anti-venom recipe with others.
The company stopped producing the serum last year and has since switched to making a rabies treatment instead.
Sanofi is believed to be negotiating for another company to produce Fav-Afrique, but these talks are not expected to be finalised before late 2016. This means that a replacement product could not be expected to reach the market for another two years, says MSF.
The absence of a broad, safe and effective anti-venom until then will translate into countless deaths, it says.
Polly Markandya of MSF said: "Most people who get bitten by a snake aren't exactly sure what kind of snake it is that bit them and so having an anti-venom that works against a variety of different species is really important.
"We are worried that without that anti-venom available, people will die unnecessarily."
Alain Bernal, a Sanofi Pasteur spokesman, said the company had offered to transfer the anti-venom technology to others, adding: "Nothing has materialised yet."
The World Health Organization says snakebites are a neglected issue that needs more attention and investment.
Each year, an estimated five million people worldwide are bitten by snakes, out of whom 100,000 die and 400,000 are permanently disabled or disfigured.
In Sub-Saharan Africa alone, 30,000 people die from snakebite every year and an estimated 8,000 undergo amputations.
The world is running out of one of the most effective snakebite treatments, putting tens of thousands of lives at risk, warn experts.