The correction to the English edition is mine: the German edition lists the drug Valeron and not methadone.
In various parts of the book and in the following book (Mein zweites Leben) it is often mentioned that methadone was not yet in use as a detoxification therapy at that time (1976).
Unfortunately I could not find all the necessary information about the Valeron. My guess is that it was the name or brand used in Germany for Diclofenac. So below is the information about Diclofenac or Voltaren.

Diclofenac, sold under the brand name Voltaren, among others, is a nonsteroidal anti-inflammatory drug (NSAID) used to treat pain and inflammatory diseases such as gout. It is taken by mouth or rectally in a suppository, used by injection, or applied to the skin. Improvements in pain last for as much as eight hours. It is also available in combination with misoprostol in an effort to decrease stomach problems. […]

“Diclofenac.” Wikipedia, Wikimedia Foundation, 15 March 2022, en.wikipedia.org/wiki/Diclofenac.

Resources:

• Diclofenac on Wikipedia
• Diclofenac on Drugs.com
• To see what happens to the brain on drugs play to the game “Mouse Party” | Genetic Science Learning Center. “Mouse Party.” Learn.Genetics. August 30, 2013. Accessed December 25, 2021. https://learn.genetics.utah.edu/content/addiction/mouse/.