Eric the chihuahua was stolen from a lorry yard in Alfreton, and his owner says he urgently needs his medication.
Owner Josh Fothergill from South Normanton told the BBC his two dogs were wandering around the EWS yard on Meadow Lane Industrial Estate at about 9.25pm on Tuesday May 14 when he saw a van turning around nearby.
Mr Fothergill, 49, said: "I saw a van coming down the road, he turned around looking sheepish and I said 'what are you looking at?'
"I asked if he'd seen a little brown dog and he said 'yeah, up by that fence' but looking at the CCTV later you can see them running round chasing the dogs.
"They must have had him with them while I spoke to them, that was why they were so nervous."
Mr Fothergill has had Eric for six years and said he had been "everywhere" with him.
He said when Eric has a fit he cannot stand up and "you have to be really careful with him".
"With all the stress that has gone off I would not be surprised if he had had a fit," he said.
"He means a lot to me, like a member of the family."
If you have any information, please call police on 101.