Use of E... Diagnosis codes for medicare

Could anyone tell me if Medicare allows to use Diagnosis codes that start with an E or any other letter as primary Dx. I heard we can only use it as second or third, but not primary. If there is something that states that we can use it as primary I need to know.

Thank you in advance for the help.

E codes

ICD-9-CM diagnosis codes that describe circumstances of injury (E codes) are never primary and Medicare does not want on claim forms. Can use as secondary code for Liability or Workers Comp cases.