The alleged attack on the neighbourhood of Jobar in the capital Damascus comes a week after the Syrian government sent a letter to the United Nations claiming it had evidence that rebel groups were planning a toxic gas attack in the same area.
Reuters could not independently verify the footage or the claims due to security restrictions on reporting in Syria.
Activists from the opposition "Jobar Revo" group posted the video on YouTube of a man being treated with oxygen and being injected by medics. A voice off-screen said Thursday's date and that there was "a poison attack in Jobar."
Another opposition group, the Syrian Revolutionary Coordinators Union, said that all those affected by the gas were "in a good condition". There has been on-off fighting between rebels and government forces in Jobar this year.
In a letter dated March 25 and circulated by the United Nations this week, Syria's U.N. envoy, Bashar Ja'afari, said his government had intercepted communications between "terrorists" that showed a man named Abu Nadir was secretly distributing gas masks in the rebel-held Jobar area.
Ja'afari said in the letter addressed to U.N. Secretary-General Ban Ki-moon and the U.N. Security Council that this information "confirms that armed terrorist groups are preparing to use toxic gas in Jobar quarter and other areas, in order to accuse the Syrian government of having committed such an act of terrorism."
A U.N. inquiry found in December that sarin gas had likely been used in Jobar in August and in several other locations, including in the rebel-held Damascus suburb of Ghouta, where hundreds of people were killed.
The inquiry was only looking at whether chemical weapons were used, not who used them. The Syrian government and the opposition have each accused the other of using chemical weapons, and both have denied it.
The Ghouta attack sparked global outrage and a U.S. threat of military strikes, which was dropped after Assad pledged to destroy his chemical weapons.
But the Syrian government failed to meet a February 5 deadline to move all of its declared chemical substances and precursors, some 1,300 tonnes, out of the country. Syria has since agreed to a new timetable to remove the weapons by late April.
Syria's three-year civil war has killed more than 150,000 people, a third of them civilians, and caused millions to flee.