Gardaí and members of the National Bureau of Criminal Investigation have raided premises in Clare and Tipperary owned by two controversial former doctors.
A Garda spokesman said the operation was targetting the activities of Dr Paschal Carmody and Dr William Howard Porter, about whom a number of complaints had been recieved.
Up to 40 gardaí, BCI staff and Irish Medicines Board personnel seized medicines and medical equipment during searches of Dr Porter's properties at Marine Village, Ballina, and Dr Carmody's private house at Ballycuggaran and treatment centres at Tinerana House.
Dr Carmody's East Clinic in Killaloe was also raided.
Both of the men have been struck off medical registers.
Dr Carmody was found guilty of professional misconduct earlier this year. While banned from practising as a doctor, he is not restricted from working in alternative medicine.
Dr Porter was struck off as an ophthalmologist in the US for gross negligence but has continued to offer alternative treatments in Ireland.
Gardaí received several complaints last year from families of former patients who had paid up to €20,000 for cancer treatment.