On one occasion Dogen said,

In the assembly of Zen master Bussho, there was a monk who, when he was sick, wanted to eat meat. The master allowed him to do so. One night the master himself went to the infirmary and saw the sick monk eating meat in the dim lamplight. A demon was clinging to the monk’s head, eating the meat. Although the monk thought he was putting it into his own mouth, it was not him, but the demon who was eating. After that whenever a monk fell ill, the master allowed him to eat meat because he knew he was possessed by demons. 

Thinking about this story, we must carefully consider whether to allow it or not. There was also an instance of eating meat in the assembly of Goso Hoen. Whether allowing or prohibiting it, all the ancient masters surely had their own deep considerations.