A nurse who went missing after taking part in a clinical trial for malaria is believed to have travelled north from his Southampton home.
Matthew Lloyd, 35, who works at the city's general hospital, failed to attend treatment in Oxford on Thursday as part of the trial.
He phoned in sick to work before his meeting and has not been seen since.
Police think Mr Lloyd, who needs life-saving medication linked to the trial, has travelled to London in recent days.
Officers have also tracked his movements to Birmingham and Milton Keynes.
As well as having connections with Oxford for the clinical trial, Mr Lloyd is known to have links with Cornwall and the Avon and Somerset area.
Det Insp Dave Jackson, of Hampshire police, said: "Our concerns for Matthew's health are increasing because he needs urgent medication, which he does not have with him.
"Even the smallest detail could prove crucial in finding Matthew so he can receive his life-saving medication in time.
"I would like to thank everyone who has given information to the police so far.
"Further enquiries are continuing into the circumstances surrounding these possible leads."