Definition of adverse:

in an opposing direction

Synonyms of adverse:

contrary, detrimental, harmful, unfavourable, negative, unfavorable, disadvantageous, hostile, inimical

Opposite/Antonyms of adverse:

favorable, helpful, good, fortunate, anodyne, advantageous, propitious, positive, harmless, medical

Sentence/Example of adverse:

The adverse effects of iron tablets are mainly feeling sick, abdominal pain, diarrhoea or constipation.