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.

When to seek immediate medical care

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.

When to make a doctor's appointment

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.