The incident happened at the Shoreham Pharmacy, Shoreham Street, at around 4pm on Monday, March 11.
Pharmacy manager Mubashir Ahmed said the member of staff was ‘helpless’ as the man helped himself to around £200 from the till.
Mr Ahmed said: “A guy came in asked for paraphernalia and distracted one member of staff who went downstairs, leaving another member of staff in the shop, who is a pregnant woman, and he entered some digits into the till and took some cash.
“The member of staff couldn’t do anything but watch him – she was helpless and it’s left her some mental scars.
“She has taken a few days off as it’s really upset her, which is the last thing she needs when she is in the latter stages of her pregnancy."
Mr Ahmed said staff at the pharmacy ‘just wanted to find those responsible’.
Anyone with any information should call police on 101, quoting incident number 598 of March 11.