Doctors Subpoenaed for Corey Haim's Rx

Approximately 20 doctors received subpoenas yesterday in connection with the Corey Haim death investigation ... law enforcement tells TMZ.

The names of the doctors appear on various prescriptions written for Haim.

We've learned the doctors are from L.A., Orange and Ventura counties ... all in Southern California.

The L.A. County Coroner's Office served the subpoenas -- investigators want to review medical records to review treatment and prescriptions.

Law enforcement tells TMZ seven of the doctors say they never saw Corey Haim or prescribed medications for him. Investigators believe the prescriptions for the seven docs may be linked to the fraudulent prescription ring currently under investigation by the State Attorney General.

And, we've learned, 2 - 3 more doctors in the Haim investigation will be subpoenaed by week's end.