Most mortiferous means highly deadly, fatal or poisonous. Some of the antonyms for this word include constructive, beneficial, healthy, and life-giving. Constructive refers to something that adds value, promotes growth, or improves a situation. Beneficial refers to an action or substance that is advantageous, helpful or profitable. Healthy refers to something that promotes well-being, physical fitness or mental wellness. Lastly, life-giving refers to something that contributes to vitality, existence or sustains life. These antonyms are critical for a better understanding of the meaning of the word mortiferous in its opposite or contrasting meanings.