When weight is lower than what is considered healthy for a person's height, or when body mass index is less than 18.5.

Focus on gaining weight safely over time. Eat a variety of nutritious foods. Follow a healthcare professional's advice on how many calories to take in each day.

See a healthcare professional right away for:

• Loss of more fluid than is taken in, also called dehydration.
• Not being able to eat or drink, or refusing to eat or drink, for days at a time.
• Falls due to weakness or dizziness.
• Severe, ongoing vomiting or diarrhea.
• Bones that stand out under the skin.
• Hollow cheeks or sunken eyes.
• Confusion.
• Lack of response when spoken to or touched.

See a healthcare professional for underweight linked with:

• Sudden weight loss.
• Loss of 5% to 10% of body weight within 3 to 6 months.
• Lack of appetite or becoming full quickly.
• Trouble chewing and swallowing.
• Feelings of dizziness, weakness or tiredness.
• Depression.
• Not being able to stay warm.
• Fluid buildup in the feet and legs.
• Changes in bowel habits.
• Trouble becoming pregnant.
• A menstrual period that becomes irregular or stops happening before menopause has started.
• Use of too much alcohol or use of illicit drugs.