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Side Effects


Clinical Trials Experience

Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared to rates in the clinical trials of another drug and may not reflect the rates observed in clinical practice.

A total of 1346 subjects were treated with at least one dose of HETLIOZ, of which 139 were treated for > 26 weeks and 93 were treated for > 1 year.

A 26-week, parallel-arm placebo-controlled study (Study 1) evaluated HETLIOZ (n=42) compared to placebo (n=42) in patients with Non-24. A randomized-withdrawal, placebo-controlled study of 8 weeks duration (Study 2) also evaluated HETLIOZ (n=10), compared to placebo (n=10), in patients with Non-24.

In placebo-controlled studies, 6% of patients exposed to HETLIOZ discontinued treatment due to an adverse event, compared with 4% of patients who received placebo.

Table 1 shows the incidence of adverse reactions from Study 1.

Table 1: Adverse Reactions in Study 1

  HETLIOZ N=42 Placebo N=42
Headache 17 % 7 %
Alanine aminotransferase increased 10 % 5 %
Nightmare/abnormal dreams 10 % 0 %
Upper respiratory tract infection 7 % 0 %
Urinary tract infection 7 % 2 %
*Adverse reactions with an incidence > 5% and at least twice as high on HETLIOZ than on placebo are displayed.

Read the Hetlioz (tasimelteon capsules) Side Effects Center for a complete guide to possible side effects


Strong CYP1A2 Inhibitors (e.g., fluvoxamine)

Avoid use of HETLIOZ in combination with fluvoxamine or other strong CYP1A2 inhibitors because of a potentially large increase in tasimelteon exposure and greater risk of adverse reactions [see CLINICAL PHARMACOLOGY] .

Strong CYP3A4 Inducers (e.g., rifampin)

Avoid use of HETLIOZ in combination with rifampin or other CYP3A4 inducers because of a potentially large decrease in tasimelteon exposure with reduced efficacy [see CLINICAL PHARMACOLOGY] .

Drug Abuse And Dependence

Controlled Substance

Tasimelteon is not a controlled substance under the Controlled Substances Act.


Tasimelteon did not produce any abuse-related signals in animal behavioral studies. Rats did not self-administer tasimelteon, suggesting that the drug does not have rewarding properties. There were also no signs or symptoms indicative of abuse potential in clinical studies with HETLIOZ.


Discontinuation of HETLIOZ in humans following chronic administration did not produce withdrawal signs. HETLIOZ does not appear to produce physical dependence.

This monograph has been modified to include the generic and brand name in many instances.

Last reviewed on RxList: 3/20/2017

Side Effects

Report Problems to the Food and Drug Administration


You are encouraged to report negative side effects of prescription drugs to the FDA. Visit the FDA MedWatch website or call 1-800-FDA-1088.

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