L-Phenylalanine This natural amino acid is a powerful, non-addictive analgesic, found in clinical research to be as effective in relieving pain as morphine or opiates. It seems to be responsible for increasing the activity of brain endorphins believed to be involved in the analgesic response. Often helps pets have superior responses to other types of pain treatment. May also help alleviate depression from long term pain or illness and reduce some seizure activity.