Four people had to be treated at London Bridge Station after being taken ill in what was initially feared to be a chemical incident.
A London Ambulance Service spokeswoman said it was called at 08:40 GMT to reports of a patient needing treatment.
London Fire Brigade sent crews but failed to detect high readings of any chemicals.
British Transport Police said it was a minor incident and restrictions at the station were minimal.
The ambulance service spokeswoman said three of the patients were taken to hospital after complaining of feeling "unwell, dizzy, and hot".
The Metropolitan Police said: "At this stage there is nothing to link the four people taken unwell."
A London Fire Brigade spokeswoman said: "Firefighters worked with the police to carry out a sweep of the scene and quickly established that the area was safe.
"It's not thought the unwell people were linked in any way, it's likely it was just a coincidence."
Part of Tooley Street was closed but has since reopened. The station is open and trains are running as normal.