He conducted human medical experimentation at Auschwitz and introduced the procedure there of injecting lethal doses of phenol directly into the hearts of prisoners.

For example, on 29 August 1942 in Auschwitz, he and Josef Klehr selected 746 prisoners to be gassed under the guise of fighting a typhus epidemic.

[5] He conducted human medical experimentation at Auschwitz, along with Helmut Vetter and Eduard Wirths,[7] where he operated in "Block 21" and was paid by the Bayer pharmaceutical subsidiary of IG Farben to test experimental drugs against typhus and tuberculosis (TB).

[4][9] The practice may have been used to cover up the results of experimental medical procedures, including surgery, that he and other doctors at the camp carried out despite not having surgical qualifications.

[7][10] In 1942, he gave Bayer associate Helmut Vetter approval to test TB drug Rutenol, an arsenic acid derivative.

On 10 February 1945, he left the camps to serve as a surgeon for the 9th SS Panzer Division Hohenstaufen.

After becoming aware of what he'd done, U.S. officials transferred him to a court they had established in the former Dachau concentration camp to stand trial.

[13] Multiple witnesses said Entress and another SS physician, Waldemar Wolter, participated in the selections of approximately 3000 prisoners.