A 35-year-old man who was arrested smuggled tobacco, a lighter and cannabis into a police cell this month, it has been revealed.
The incident, which happened at Worcester police station, is now being looked at by the Independent Police Complaints Commission.
The police watchdog said it wanted to know how a man with a history of self-harm was able to take a lighter in.
Commissioner Rebecca Marsh said there were several issues that concerned her.
'Duty of care'
The police watchdog said it would be independently investigating after the man in custody smuggled items, including tobacco, a lighter and cannabis, into the cell on 6 August.
The commissioner added: "There are several issues that concern me about this incident, such as the extent of the searches made, and the risk assessment of somebody with markers on the police computer.
"Not only have illegal drugs been taken into the cell with the detainee, but a man with a history of previous self-harm was also able to take a lighter in with him.
"Our investigation will address the actions of the officers and establish how such a situation arose."
She added it would also look at West Mercia Police's capacity "to safeguard those they have a duty of care towards to be properly assessed".