Some 150 customers and staff were evacuated from the store, while people were cleared from the surrounding area.
A spokesperson for West Midlands Ambulance Service said: "Sainsburys and other nearby buildings were evacuated after several people inside the supermarket experienced respiratory problems.
"A total of seven patients were assessed by the ambulance service after displaying minor respiratory problems.
"Two women received treatment on scene before being conveyed by ambulance, in a stable condition, to Queen Elizabeth Hospital Birmingham for further checks.The remaining five patients were later discharged on scene.
"A number of other people at the scene, who were described as 'worried but well', received advice from the team of ambulance staff."
West Midlands Fire Service, which described the event on its website as a "hazardous substances incident", has deployed "air monitoring equipment" to try to establish the cause.
A spokesperson said: "We are working closely with the store management, as well as our ambulance and police colleagues and Birmingham City Council."Around 150 people were evacuated from the store.
"A nearby college was not evacuated, but advised to keep windows closed as a precaution."
The police urged anyone nearby who was feeling unwell to seek medical advice.
A Sainsbury's spokesperson said: "Emergency services attended our Longbridge store this morning. Safety is our highest priority and the store is closed while we assist the police with their investigations."
The fire service said that the High Street, which had been blocked off as part of the emergency cordon, would shortly reopen, but the Sainsbury's store would "remain closed for the time being" pending their ongoing investigations.