Reviewed on 3/8/2023

What Is Tasimelteon and How Does It Work?

Tasimelteon is a prescription medication used to treat the symptoms of Non-24-hour Disorder and Smith-Magenis Syndrome (SMS).

  • Tasimelteon is available under the following different brand names: Hetlioz

What Are Side Effects Associated with Using Tasimelteon?

Common side effects of Tasimelteon include:

Serious side effects of Tasimelteon include:

  • hives,
  • difficulty breathing,
  • swelling of the face, lips, tongue, or throat, and
  • pain or burning while urinating

Rare side effects of Tasimelteon include:

  • none

Seek medical care or call 911 at once if you have the following serious side effects:

  • Severe headache, confusion, slurred speech, arm or leg weakness, trouble walking, loss of coordination, feeling unsteady, very stiff muscles, high fever, profuse sweating, or tremors;
  • Serious eye symptoms such as sudden vision loss, blurred vision, tunnel vision, eye pain or swelling, or seeing halos around lights;
  • Serious heart symptoms include fast, irregular, or pounding heartbeats; fluttering in the chest; shortness of breath; sudden dizziness, lightheadedness, or passing out. 

This is not a complete list of side effects and other serious side effects or health problems that may occur because of the use of this drug. Call your doctor for medical advice about serious side effects or adverse reactions. You may report side effects or health problems to FDA at 1-800-FDA-1088.

What Are the Dosages of Tasimelteon?

Adult and pediatric dosage


  • 20 mg (Hetlioz)

Oral suspension

  • 4 mg/mL (Hetlioz Lq)

Non-24-Hour Disorder

Adult dosage

  • 20 mg orally per day taken before bedtime, at the same time every night

Smith-Magenis Syndrome

Adult dosage

  • Capsule: 20 mg orally 1 hour before bedtime, at the same time every night

Pediatric dosage


  • Aged above 16 years
    • 20 mg orally 1 hr before bedtime, at the same time every night

Oral suspension

  • Aged below 3 years: No data available
  • 3-15 years
    • Weighing below 28 kg: 0.7 mg/kg orally 1 hr before bedtime
    • Weighing above 28 kg: 20 mg orally 1 hr before bedtime

Dosage Considerations – Should be Given as Follows: 

  • See “Dosages”

What Other Drugs Interact with Tasimelteon?

If your medical doctor is using this medicine to treat your pain, your doctor or pharmacist may already be aware of any possible drug interactions and may be monitoring you for them. Do not start, stop, or change the dosage of any medicine before checking with your doctor, health care provider, or pharmacist first

  • Tasimelteon has severe interactions with no other drugs.
  • Tasimelteon has serious interactions with at least 24 other drugs.
  • Tasimelteon has moderate interactions with at least 82 other drugs.
  • Tasimelteon has minor interactions with the following drugs:
    • acetazolamide
    • anastrozole
    • cyclophosphamide
    • larotrectinib
    • ribociclib

This information does not contain all possible interactions or adverse effects. Visit the RxList Drug Interaction Checker for any drug interactions. Therefore, before using this product, tell your doctor or pharmacist about all your products. Keep a list of all your medications with you and share this information with your doctor and pharmacist. Check with your healthcare professional or doctor for additional medical advice, or if you have health questions or concerns.

What Are Warnings and Precautions for Tasimelteon?


  • None

Effects of drug abuse

  • None

Short-Term Effects

  • See “What Are Side Effects Associated with Using Tasimelteon?”

Long-Term Effects

  • See “What Are Side Effects Associated with Using Tasimelteon?”


  • After administration, advise patients to limit activity to prepare for going to bed
  • May potentially impair the performance of activities requiring complete mental alertness
  • Drug interaction overview
    • Tasimelteon is a CYP3A4 and CYP1A2 substrate
  • Smokers
    • Smoking is a moderate CYP1A2 inducer
    • Tasimelteon exposure decreased by ~40% in smokers compared with nonsmokers
  • Strong CYP1A2 Inhibitors
    • Avoid coadministration
    • Fluvoxamine or other strong CYP1A2 inhibitors may potentially increase tasimelteon exposure and increase the risk of adverse reactions
  • Strong CYP3A4 inducers
    • Avoid coadministration
    • Rifampin or other CYP3A4 inducers may potentially decrease tasimelteon exposure and reduced efficacy
  • Beta-adrenergic receptor antagonists
    • Beta-adrenergic receptor antagonists have been shown to reduce production of melatonin via specific inhibition of beta-1 adrenergic receptors
    • Nighttime administration of beta-adrenergic receptor antagonists may reduce tasimelteon efficacy

Pregnancy and Lactation

  • Available postmarketing case reports use in pregnant females are not sufficient to evaluate drug-associated risk of major birth defects, miscarriage or adverse maternal or fetal outcomes
  • Lactation
    • No data available on drug presence or its metabolites in human or animal milk, effects on breastfed infants, effects on milk production

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