HETLIOZ: Uses, Dosage & Side Effects

What Is HETLIOZ and What Is It Used For?

HETLIOZ contains the active substance tasimelteon, a selective dual agonist of the melatonin MT1 and MT2 receptors. Unlike melatonin supplements available over the counter in many countries, tasimelteon is a specifically engineered pharmaceutical compound with well-characterized pharmacokinetics, a defined dose-response relationship, and regulatory approval based on rigorous clinical trial evidence. It was developed by Vanda Pharmaceuticals and received its first approval from the U.S. Food and Drug Administration (FDA) on January 31, 2014, making it the first drug ever approved specifically for the treatment of Non-24-Hour Sleep-Wake Disorder. In July 2015, the European Medicines Agency (EMA) granted marketing authorization for HETLIOZ in the European Union.

Non-24-Hour Sleep-Wake Disorder (Non-24) is a serious chronic circadian rhythm disorder that occurs when the body’s master biological clock, located in the suprachiasmatic nucleus (SCN) of the hypothalamus, fails to synchronize with the external 24-hour light-dark cycle. In sighted individuals, the SCN receives light information from the retina through specialized retinal ganglion cells containing the photopigment melanopsin. This photic input is the most powerful zeitgeber (time-giver) for circadian entrainment, resetting the internal clock each day to match the 24-hour environment. In totally blind individuals who lack any light perception, this photic entrainment pathway is absent, and the endogenous circadian period — which in most humans runs slightly longer than 24 hours (averaging approximately 24.2 hours) — gradually drifts out of synchrony with the external day.

The consequences of this desynchronization are profound. As the internal clock drifts, sleep and wake times shift progressively later day by day. Patients experience recurring cycles of good sleep alignment (when their internal clock happens to coincide with their desired bedtime) alternating with periods of severe insomnia, excessive daytime sleepiness, and functional impairment (when the internal clock is misaligned). These cycles typically recur every few weeks to months. The impact on daily life, employment, social functioning, and mental health can be devastating. Studies suggest that up to 70% of totally blind individuals may have Non-24, making it a surprisingly common yet widely underdiagnosed condition.

Tasimelteon addresses this problem by providing a non-photic circadian entrainment signal through the melatonin receptor pathway. The MT1 and MT2 receptors are both expressed in the SCN and play complementary roles in circadian regulation. The MT1 receptor is primarily involved in the acute inhibition of SCN neuronal firing, which promotes the onset of sleep. The MT2 receptor is thought to be the primary mediator of circadian phase-shifting, the ability to advance or delay the internal clock. Tasimelteon has greater binding affinity for the MT2 receptor (Ki = 0.35 nM) compared to the MT1 receptor (Ki = 0.3 nM), with a selectivity profile considered optimal for circadian re-entrainment. By activating both receptors, tasimelteon provides both an acute sleep-promoting signal and a sustained phase-shifting effect that gradually pulls the free-running circadian clock into alignment with a 24-hour cycle.

The pivotal clinical evidence for HETLIOZ came from two randomized, double-blind, placebo-controlled phase III trials:

• SET Trial (Safety and Efficacy of Tasimelteon): This study enrolled 84 totally blind adults with Non-24. Patients received either tasimelteon 20 mg or placebo daily one hour before bedtime. The primary endpoints were circadian entrainment as measured by urinary 6-sulphatoxymelatonin (aMT6s, the principal melatonin metabolite) and urinary cortisol rhythms. Tasimelteon demonstrated statistically significant entrainment of both the melatonin and cortisol circadian markers compared with placebo, and also showed significant improvements in total nighttime sleep time and total daytime sleep time.
• RESET Trial (Randomized Withdrawal Study of the Efficacy of Tasimelteon): This withdrawal study enrolled 20 patients with Non-24 who had been entrained on tasimelteon during an open-label run-in period. Patients were randomized to continue tasimelteon or switch to placebo. Those switched to placebo showed a loss of circadian entrainment, confirming that the therapeutic benefit of tasimelteon requires continued daily administration.

In the EU, HETLIOZ has also been evaluated and approved for the treatment of Non-24-Hour Sleep-Wake Disorder in totally blind adults. The EMA assessment was based on the same pivotal trials (SET and RESET) along with supportive long-term safety data. The Committee for Medicinal Products for Human Use (CHMP) concluded that the benefits of HETLIOZ outweigh its risks in this population. Long-term follow-up data from open-label extension studies have demonstrated that tasimelteon maintains circadian entrainment over periods exceeding 3 years, with a stable and well-tolerated safety profile.

What Should You Know Before Taking HETLIOZ?

Contraindications

The primary contraindication to HETLIOZ use is known hypersensitivity to tasimelteon or to any of the excipients in the formulation. The capsule contains the following inactive ingredients: lactose monohydrate, microcrystalline cellulose, croscarmellose sodium, colloidal silicon dioxide, magnesium stearate, and a gelatin capsule shell with colorants (titanium dioxide, FD&C Blue No. 1, FD&C Blue No. 2, FD&C Red No. 40, and black iron oxide for the capsule imprint). Patients with known allergies to any of these components should not take HETLIOZ.

Additionally, concomitant use of HETLIOZ with fluvoxamine or other strong CYP1A2 inhibitors is contraindicated due to the substantial increase in tasimelteon plasma concentrations that may result. Fluvoxamine is the most potent known inhibitor of CYP1A2 and has been shown to increase tasimelteon exposure by approximately 7-fold in pharmacokinetic studies, dramatically altering the drug’s safety profile.

Warnings and Precautions

Before starting HETLIOZ, discuss the following with your healthcare provider:

• Hepatic impairment: Tasimelteon is extensively metabolized by the liver, primarily through CYP1A2 and CYP3A4 pathways. In patients with mild to moderate hepatic impairment, no dose adjustment is required, but caution is advised. HETLIOZ has not been studied in patients with severe hepatic impairment and is generally not recommended in this population due to the potential for significantly increased drug exposure.
• Smoking status: Tobacco smoking is a potent inducer of CYP1A2. In smokers, the metabolism of tasimelteon is accelerated, leading to reduced plasma concentrations and potentially diminished efficacy. Patients who smoke should inform their healthcare provider, as the effectiveness of HETLIOZ may be reduced. Conversely, patients who stop smoking during treatment may experience increased tasimelteon levels.
• Lactose intolerance: HETLIOZ capsules contain lactose monohydrate. Patients with rare hereditary problems of galactose intolerance, total lactase deficiency, or glucose-galactose malabsorption should not take this medicine.
• Concomitant medications: Inform your doctor about all prescription and non-prescription medications, herbal supplements, and dietary supplements you are taking. Special attention should be given to CYP1A2 inhibitors and inducers, CYP3A4 inhibitors and inducers, and beta-adrenergic receptor antagonists (beta-blockers), which may affect melatonin signaling.

Children and Adolescents

HETLIOZ is not approved for use in children under 18 years of age for the treatment of Non-24-Hour Sleep-Wake Disorder. However, it is worth noting that the FDA has approved a separate oral liquid formulation of tasimelteon (HETLIOZ LQ) for the treatment of Non-24 in pediatric patients with blindness. The pediatric formulation allows for weight-based dosing and was approved based on pharmacokinetic modeling, safety data, and supportive efficacy evidence. If a pediatric patient is being considered for tasimelteon treatment, the appropriate pediatric formulation and dosing should be used under the supervision of a specialist in pediatric sleep medicine.

The safety and efficacy of HETLIOZ capsules in the pediatric population for Non-24 have not been established through dedicated pediatric clinical trials with this specific formulation. The 20 mg hard capsule is designed for adult use. Healthcare providers should exercise clinical judgment when considering treatment options for pediatric patients with circadian rhythm disorders.

Pregnancy and Breastfeeding

There are limited data on the use of HETLIOZ during pregnancy. Animal reproductive and developmental toxicity studies conducted in rats and rabbits have shown adverse effects (including reduced fetal body weight and skeletal variations in rats, and increased post-implantation loss in rabbits) at doses producing exposures significantly higher than the recommended human dose. The relevance of these findings to humans at the therapeutic dose is uncertain. As a precautionary measure, HETLIOZ should be avoided during pregnancy unless the potential benefit to the mother clearly justifies the potential risk to the fetus. Women of childbearing potential should discuss effective contraception with their healthcare provider.

It is not known whether tasimelteon or its metabolites are excreted in human breast milk. In animal studies, tasimelteon and its metabolites were detected in rat milk. Given that many drugs are excreted in human milk and that there is a potential for adverse reactions in breastfed infants, a decision should be made whether to discontinue breastfeeding or to discontinue HETLIOZ, taking into account the importance of the drug to the mother. The relatively short elimination half-life of tasimelteon (approximately 1.3 hours) may be a consideration in this decision, although the pharmacodynamic effects on circadian rhythm extend well beyond the period of measurable plasma concentrations.

Driving and Operating Machinery

HETLIOZ may cause somnolence. The effects on the ability to drive and operate machinery have not been specifically studied. However, because HETLIOZ may impair alertness, patients should exercise caution when engaging in activities requiring full mental alertness after taking the medication. It is recommended that HETLIOZ be taken only before bedtime, and that patients limit their activities after dosing to preparations for going to sleep. For blind patients, the relevance of driving is limited, but the somnolence warning extends to the operation of any potentially dangerous equipment or machinery.

How Does HETLIOZ Interact with Other Drugs?

Unlike many medications that target receptors in the brain, tasimelteon has a clinically significant drug interaction profile due to its hepatic metabolism. Tasimelteon is primarily metabolized by cytochrome P450 enzymes CYP1A2 and CYP3A4, with CYP1A2 being the major pathway. This means that any drug, food, or substance that significantly affects the activity of these enzymes can alter tasimelteon plasma concentrations and, consequently, its efficacy or safety profile.

The most clinically important interaction is with strong CYP1A2 inhibitors. Pharmacokinetic studies have demonstrated that co-administration of fluvoxamine (a strong CYP1A2 inhibitor commonly used as an antidepressant and for obsessive-compulsive disorder) increases the area under the plasma concentration-time curve (AUC) of tasimelteon by approximately 7-fold. This dramatic increase in exposure could lead to a significant increase in the risk and severity of adverse effects. For this reason, concomitant use of HETLIOZ with fluvoxamine is contraindicated, and use with other strong CYP1A2 inhibitors should be avoided.

The following table summarizes the key drug interactions with HETLIOZ based on pharmacokinetic studies, pharmacological principles, and clinical guidance:

Major Interactions

The interaction with fluvoxamine represents the most clinically significant concern. Fluvoxamine is widely prescribed for depression, anxiety disorders, and obsessive-compulsive disorder. Patients who are currently taking fluvoxamine and who require treatment for Non-24 should discuss alternative antidepressant options with their prescribing physician before starting HETLIOZ. Other SSRIs such as sertraline, citalopram, and escitalopram have minimal CYP1A2 inhibitory activity and may be suitable alternatives.

Rifampicin (rifampin), a cornerstone of tuberculosis treatment and a potent inducer of multiple CYP enzymes, reduces tasimelteon exposure by approximately 58%. This reduction is likely sufficient to compromise the circadian entrainment effect of HETLIOZ. Patients requiring concurrent rifampicin therapy may not achieve adequate benefit from HETLIOZ, and alternative approaches to managing their Non-24 should be considered.

Minor Interactions

Moderate CYP1A2 inhibitors such as ciprofloxacin and certain herbal products may increase tasimelteon levels to a lesser degree than fluvoxamine but still warrant caution. Similarly, CYP3A4 inhibitors such as ketoconazole produce a moderate increase in exposure that may require monitoring but does not necessarily contraindicate use. Beta-adrenergic receptor blockers (beta-blockers) are known to suppress endogenous melatonin secretion through their effects on adrenergic signaling to the pineal gland. While the clinical significance of this interaction with exogenous melatonin receptor agonists is not fully established, clinicians should be aware that beta-blockers may theoretically attenuate the circadian effects of HETLIOZ.

There is no evidence that tasimelteon significantly inhibits or induces major CYP enzymes at therapeutic concentrations. Therefore, HETLIOZ is not expected to affect the metabolism of most other co-administered drugs. However, tasimelteon is a weak inhibitor of CYP2C8 and CYP2C19 in vitro, and caution may be warranted with drugs that are sensitive substrates of these enzymes, although clinically relevant interactions have not been demonstrated.

What Is the Correct Dosage of HETLIOZ?

The dosing of HETLIOZ is straightforward compared to many other medications, with a single fixed dose for all adult patients. The critical factor is not the dose itself but the consistency of timing. Because HETLIOZ works by providing a daily time cue to the circadian clock, it must be taken at the same time each evening to effectively entrain the biological rhythm. Even modest variations in dosing time can undermine the entrainment process.

Adults

The 20 mg dose was selected based on phase II dose-ranging studies that evaluated doses from 10 mg to 100 mg. The 20 mg dose demonstrated the optimal balance between efficacy (circadian entrainment and sleep improvements) and tolerability. Higher doses did not provide additional clinical benefit and were associated with a higher incidence of adverse effects. The capsule should be taken approximately one hour before the patient’s desired bedtime, and patients should prepare for bed after taking the medication.

It is important for patients to understand that HETLIOZ does not work like a conventional sleeping pill. There is no immediate sedative effect at the therapeutic dose. Instead, the medication provides a daily pharmacological time cue that, over weeks to months, gradually shifts the free-running circadian clock into alignment with the 24-hour day. Patients should be counseled to continue taking HETLIOZ consistently even if they do not notice an immediate improvement in sleep, as the entrainment process is gradual.

Elderly

Renal and Hepatic Impairment

Missed Dose

If a dose of HETLIOZ is missed, it should be skipped and the next dose should be taken at the regular scheduled time the following evening. Do not take a double dose to make up for a missed dose. Because HETLIOZ works by providing a consistent daily time cue to the circadian clock, occasional missed doses may slow the entrainment process but are unlikely to cause immediate adverse effects. However, multiple consecutive missed doses can lead to a loss of circadian entrainment, and the re-entrainment process would need to begin again. Patients should strive to take HETLIOZ every day at the same time.

Overdose

There is limited clinical experience with HETLIOZ overdose. In healthy volunteer studies, single doses up to 300 mg (15 times the recommended dose) were administered without serious adverse effects. Symptoms of overdose are expected to include exaggerated pharmacological effects, primarily somnolence (excessive drowsiness), headache, and possibly dizziness. There is no specific antidote for tasimelteon overdose. In the event of an overdose, supportive and symptomatic treatment should be provided. Gastric lavage may be considered if the ingestion is recent, although the rapid absorption of tasimelteon (peak levels within 0.5–3 hours) may limit the utility of this approach. Due to the high protein binding of tasimelteon (approximately 89–90%), dialysis is unlikely to be effective in removing the drug from the circulation.

What Are the Side Effects of HETLIOZ?

The safety profile of HETLIOZ has been evaluated in clinical trials involving a total of approximately 1,346 subjects, including 139 patients with Non-24-Hour Sleep-Wake Disorder who received tasimelteon 20 mg daily in controlled studies, and additional subjects in open-label extension studies and healthy volunteer trials. The overall tolerability of HETLIOZ has been favorable, with most adverse effects being mild to moderate in intensity and generally not leading to treatment discontinuation.

In the pivotal placebo-controlled trials (SET and RESET), the proportion of patients discontinuing treatment due to adverse effects was similar between the tasimelteon and placebo groups (approximately 3% in each). The most frequently reported adverse reactions are summarized in the frequency grid below, categorized according to standard international conventions for adverse drug reaction reporting:

The elevation in liver enzymes (ALT) observed in clinical trials was generally mild and asymptomatic, resolving either spontaneously or after discontinuation of the drug. Regular monitoring of liver function tests is not routinely required but may be considered in patients with pre-existing liver conditions or those taking other hepatotoxic medications. There were no cases of clinically significant liver injury (such as drug-induced liver injury meeting Hy’s Law criteria) reported in the clinical trial program.

Abnormal dreams and nightmares were reported more frequently in patients taking tasimelteon compared with placebo. This is pharmacologically plausible given that melatonin receptor activation can influence REM sleep architecture. These effects were generally transient and mild, and most patients who experienced them continued treatment without dose modification.

Post-marketing surveillance has not identified significant new safety signals beyond those observed in clinical trials. Long-term open-label extension studies spanning more than 3 years have confirmed the sustained tolerability of tasimelteon 20 mg daily, with no evidence of tolerance (loss of effect over time), dependence, or withdrawal symptoms upon discontinuation. This favorable long-term safety profile is particularly important given that Non-24 is a chronic condition requiring ongoing treatment.

How Should You Store HETLIOZ?

Proper storage of HETLIOZ is important to maintain the stability, potency, and safety of the medication throughout its shelf life. Tasimelteon capsules should be stored at controlled room temperature, defined as 15°C to 30°C (59°F to 86°F). Brief excursions to temperatures between 0°C and 40°C (32°F to 104°F) are permitted, as may occur during normal shipping and handling, but prolonged exposure to temperatures outside the recommended range should be avoided.

The capsules should be kept in their original container with the cap tightly closed to protect them from light and moisture. Tasimelteon is sensitive to light degradation, and exposure to direct sunlight or intense artificial light may reduce the potency of the medication. The container includes a desiccant packet to absorb moisture; this packet should be left in the container and should not be ingested. Do not transfer the capsules to a different container, pill organizer, or blister pack unless the alternative container provides equivalent protection from light and moisture.

Check the expiration date on the container before taking each dose. Do not use HETLIOZ after the expiration date printed on the label. Expired medication may have reduced potency and should be returned to a pharmacy for proper disposal. Do not flush HETLIOZ capsules down the toilet or dispose of them in household waste. Instead, follow local regulations for pharmaceutical waste disposal, or use a drug take-back program if available in your area.

Keep HETLIOZ out of reach and sight of children. The capsules are not child-resistant and could pose a risk if ingested by a child. If a child accidentally ingests HETLIOZ, contact a poison control center or seek medical attention immediately, even if the child appears well.

What Does HETLIOZ Contain?

Each HETLIOZ hard capsule contains 20 mg of tasimelteon as the active pharmaceutical ingredient. Tasimelteon is a small molecule with the chemical name (1R,2R)-2-(2,3-dihydrobenzofuran-4-yl)cyclopropanecarboxamide trans-N-[[2-(2,3-dihydro-1-benzofuranyl)cyclopropyl]methyl]propanamide. Its molecular formula is C₁₅H₁₉NO₂ and its molecular weight is 245.32 g/mol. Tasimelteon is a white to off-white crystalline powder that is slightly soluble in water and freely soluble in ethanol and methanol.

The inactive ingredients (excipients) in HETLIOZ capsules serve specific pharmaceutical functions:

• Lactose monohydrate: A commonly used pharmaceutical diluent (filler) that helps to bulk the formulation to an appropriate size for encapsulation. Patients with lactose intolerance should be aware of this ingredient, although the amount present in a single capsule is small.
• Microcrystalline cellulose: A widely used pharmaceutical excipient that serves as a binder and diluent, contributing to the structural integrity and uniformity of the capsule contents.
• Croscarmellose sodium: A superdisintegrant that promotes rapid breakdown and dissolution of the capsule contents in the gastrointestinal tract, facilitating absorption of tasimelteon.
• Colloidal silicon dioxide: A glidant that improves the flow properties of the powder blend during manufacturing, ensuring consistent and uniform capsule filling.
• Magnesium stearate: A lubricant that prevents the powder blend from sticking to manufacturing equipment during the encapsulation process.

The capsule shell is made of gelatin and contains the following colorants: titanium dioxide (E171), FD&C Blue No. 1, FD&C Blue No. 2, and FD&C Red No. 40. The capsule printing ink contains black iron oxide (E172) and shellac. The HETLIOZ capsule has a distinctive dark blue-green opaque cap and dark blue-green opaque body, printed with “VANDA 20 mg” in black ink on both the cap and the body. This distinctive appearance helps patients and healthcare providers identify the medication correctly.

HETLIOZ does not contain gluten, nuts, soy, or any animal-derived ingredients other than gelatin. Patients who follow a vegetarian or vegan diet or who have religious or cultural dietary restrictions regarding gelatin should be aware that the capsule shell is derived from animal sources (typically bovine or porcine gelatin). Alternative capsule formulations are not currently available.