Antonym: harmless. Similar words: charm, harmony, harmonize, harmonious, farm, arm, warm, farmer. Meaning: ['hɑrmfʊl /hɑːm-] 
adj. 1. injurious to physical or mental health 2. able or likely to do harm 3. causing or capable of causing harm 4. constituting a disadvantage 5. contrary to your interests or welfare 6. tending to cause great harm.
adj. 1. injurious to physical or mental health 2. able or likely to do harm 3. causing or capable of causing harm 4. constituting a disadvantage 5. contrary to your interests or welfare 6. tending to cause great harm. 
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(181) Excessive drinking is harmful to the health.
(182) It's believed the affair was potentially harmful to British aviation.