This is a single site, single dose clinical trial of intravenous (IV) ketamine for medically hospitalized adolescents and young adults with anorexia nervosa or atypical anorexia nervosa. Eating disorder symptoms will be measured pre- and post-ketamine infusion. Investigators hypothesize that ketamine will increase cognitive flexibility, making medical hospitalizations less distressing by improving the ability to learn new, positive associations with food.

Inclusion Criteria:

1. Age 16-26 years old
2. Meets diagnosis of anorexia nervosa (AN) or atypical anorexia nervosa (atypical AN) per DSM-5 criteria on admission
3. Admitted to the medical hospital for malnutrition
4. No changes to psychiatric medications for month prior to trial enrollment
5. Agree to use a highly effective form of contraception for at least two weeks following ketamine administration
6. Have an outpatient primary care provider, medical provider, or behavioral health provider or psychiatrist who is actively managing or coordinating care

Exclusion Criteria:

1. Lifetime history of any psychotic disorder
2. Moderate or severe substance use disorder
3. Pregnancy as indicated by a positive urine pregnancy test during screening, lactation, or the intention of becoming pregnant within 3 months of entry into this trial
4. Electroconvulsive therapy (ECT) or transcranial magnetic stimulation (TMS) treatment within 30 days prior to enrollment
5. Intellectual or developmental disability
6. High risk for self-harm/suicide
7. Active laxative misuse or abuse
8. Biochemical refeeding syndrome or electrolyte abnormality
9. Cardiac abnormalities identified on admission
10. Taking medications that would be unsafe to administer with ketamine
11. Other medical condition or diagnosis, physical exam finding, laboratory abnormality, or health risk that would increase the risk of adverse events, at the discretion of the investigators