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Generic name: tasimelteon
Brand names: Hetlioz, Hetlioz LQ
Dosage forms: oral capsule (20 mg); oral suspension (4 mg/mL)
Drug class: Miscellaneous anxiolytics, sedatives and hypnotics

Medically reviewed by  A Ras MD.

What is tasimelteon used for?

Tasimelteon is a prescription medicine that is used to treat sleep problems.

Before taking tasimelteon, tell your doctor:

  • If you are allergic to tasimelteon; any part of this medicine; or any other drugs, foods, or substances. Tell your doctor about the allergy and what signs you had.
  • If you have liver disease.
  • If you are taking rifampin.
  • If you are taking fluvoxamine.

This is not a list of all drugs or health problems that interact with tasimelteon.

Tell your doctor and pharmacist about all of your drugs (prescription or OTC, natural products, vitamins) and health problems. You must check to make sure that it is safe for you to take tasimelteon with all of your drugs and health problems. Do not start, stop, or change the dose of any drug without checking with your doctor.


HETLIOZ (contains tasimelteon) a melatonin receptor agonist, chemically designated as (1R, 2R)-N-[2-(2,3-dihydrobenzofuran-4-yl)cyclopropylmethyl]propanamide, containing two chiral centers. The molecular formula is C15H19NO2, and the molecular weight is 245.32. The structural formula is:


Tasimelteon is a white to off-white crystalline powder. It is very slightly soluble in cyclohexane, slightly soluble in water and 0.1 N hydrochloric acid, and freely soluble or very soluble in methanol, 95% ethanol, acetonitrile, isopropanol, polyethylene glycol 300, propylene glycol and ethyl acetate.

HETLIOZ capsules are intended for oral administration. Each capsule contains 20 mg of tasimelteon and the following inactive ingredients: colloidal silicon dioxide, croscarmellose sodium, lactose anhydrous, magnesium stearate, and microcrystalline cellulose. Each hard gelatin capsule consists of FD&C Blue #1, FD&C Red #3, and FD&C Yellow #6, gelatin, and titanium dioxide.

HETLIOZ LQ oral suspension contains 4 mg of tasimelteon per mL of suspension and the following inactive ingredients: ascorbic acid, cherry flavor, mannitol, microcrystalline cellulose/carboxymethylcellulose sodium, polysorbate 80, sodium benzoate, sodium chloride, sucrose, sucralose, and water.

Mechanism of Action

The mechanism by which tasimelteon exerts its therapeutic effect in patients with Non-24 or nighttime sleep disturbances in SMS is unclear. However, tasimelteon is an agonist at melatonin MT1 and MT2 receptors which are thought to be involved in the control of circadian rhythms.

What are some things I need to know or do while I take tasimelteon?

  • Tell all of your health care providers that you take tasimelteon. This includes your doctors, nurses, pharmacists, and dentists.
  • Avoid driving and doing other tasks or actions that call for you to be alert after you take tasimelteon. You may still feel sleepy the day after you take tasimelteon. Avoid these tasks or actions until you feel fully awake.
  • It may take several weeks to see the full effects.
  • Talk with your doctor before you drink alcohol or use other drugs and natural products that slow your actions.
  • If you smoke, talk with your doctor.
  • If you are 65 or older, use tasimelteon with care. You could have more side effects.
  • Tell your doctor if you are pregnant, plan on getting pregnant, or are breast-feeding. You will need to talk about the benefits and risks to you and the baby.

How is tasimelteon taken?

Use tasimelteon as ordered by your doctor. Read all information given to you. Follow all instructions closely.

  • Take tasimelteon before bedtime at the same time every night.
  • Take tasimelteon without food.
  • Swallow whole. Do not chew, break, or crush.
  • After you take tasimelteon, only do things needed to get ready for bed.

What do I do if I miss a dose?

  • Skip the missed dose and go back to your normal time.
  • Do not take 2 doses at the same time or extra doses.

What are the side effects of tasimelteon that I need to call my doctor about immediately?

WARNING/CAUTION: Even though it may be rare, some people may have very bad and sometimes deadly side effects when taking a drug. Tell your doctor or get medical help right away if you have any of the following signs or symptoms that may be related to a very bad side effect:

  • Signs of an allergic reaction, like rash; hives; itching; red, swollen, blistered, or peeling skin with or without fever; wheezing; tightness in the chest or throat; trouble breathing, swallowing, or talking; unusual hoarseness; or swelling of the mouth, face, lips, tongue, or throat.
  • Signs of a urinary tract infection (UTI) like blood in the urine, burning or pain when passing urine, feeling the need to pass urine often or right away, fever, lower stomach pain, or pelvic pain.

What are some other side effects of tasimelteon?

All drugs may cause side effects. However, many people have no side effects or only have minor side effects. Call your doctor or get medical help if any of these side effects or any other side effects bother you or do not go away:

  • Headache.
  • Bad dreams.
  • Signs of a common cold.

These are not all of the side effects that may occur. If you have questions about side effects, call your doctor. Call your doctor for medical advice about side effects.

You may report side effects to the FDA at 1-800-332-1088. You may also report side effects at https://www.fda.gov/medwatch.

If overdose is suspected:

If you think there has been an overdose, call your poison control center or get medical care right away. Be ready to tell or show what was taken, how much, and when it happened.

How do I store and/or throw out tasimelteon?

  • Store at room temperature protected from light. Store in a dry place. Do not store in a bathroom.
  • Keep all drugs in a safe place. Keep all drugs out of the reach of children and pets.
  • Throw away unused or expired drugs. Do not flush down a toilet or pour down a drain unless you are told to do so. Check with your pharmacist if you have questions about the best way to throw out drugs. There may be drug take-back programs in your area.




20 mg Bottle (Composite)



48 mL Bottle Label


48 mL Carton Label



158 mL Bottle Label



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