Colorado health officials have warned patients of dentist Stephen Stein that his reuse of needles and syringes may have exposed them to hepatitis B, hepatitis C and HIV, the Legal Examiner reports.

An investigation showed that Stein’s practice, which is based in Denver and Highland Ranch, regularly reused syringes and needles, often over a period of days, to inject drugs into patients’ intravenous (IV) lines during surgery. Tiny quantities of blood in the IV line can potentially spread an infectious disease to other patients as a result of such reuse.

The Colorado Department of Public Health and Environment has sent notices to 8,000 of Stein’s patients, including those treated as far back as September 1999. Patients who received IV medications are advised to get tested for HIV, hepatitis B and hepatitis C. Those who did not receive such medications are not at risk.

Stein is no longer practicing as a result of a 2011 agreement with the Colorado Board of Dental Examiners, which voted to suspend his license for willfully violating state law on dental practice and endangering the health of his patients. He is also under investigation for prescription fraud.