Medicines available in high street chemists could be being used by rapists to sedate victims, police have said.
Drug-assisted sexual assaults often involve spiked drinks
Traces of over-the-counter drugs have been found in the systems of female victims of drug-assisted sex attacks.
No evidence has yet been found that they were administered by attackers but police are examining the possibility.
The investigation forms part of Operation Matise which is also looking into the use of illegal drugs like GHB or Rohypnol in sexual assaults.
The operation is working with eight police forces around the UK to gather information from rape victims who say they were drugged.
Acting Det Insp Julie Sproson, of Derbyshire police, said some prescribed or over-the-counter drugs had "a lethargic and sleepy effect".
"When mixed with alcohol they will give a greater effect," she said.
It was not yet possible to say that offenders had definitely put the over-the-counter drugs into the drinks of their victims, she stressed.
In the eight force areas, rape victims who say they have been drugged give urine and blood samples for analysis as well as filling in a questionnaire about any over-the-counter drugs they may have taken themselves.
The year-long project has another six months to run before a final report will be published.
The team aims to provide a clear idea about the prevalence of drugs in rape cases.
Details of developments in the operation were revealed at a meeting for female police officers ahead of Tuesday's Police Federation conference in Blackpool, Lancashire.