What the label lists and what reports show
Tasimelteon side effects
Tasimelteon's FDA label: “The most common adverse reactions (incidence >5% and at least twice as high on HETLIOZ than on placebo) were headache, increased alanine aminotransferase, nightmares or unusual dreams, and upper respiratory or urinary tract infection ( 6.1 ) To report …” FAERS holds 5,770 reports naming it through July 30, 2026; the reaction reported most often, leaving out the conditions it is used for, product-use terms and outcomes, is insomnia (728 reports).
- Generic listed
Check it at the source: the tasimelteon FDA label on DailyMed (effective August 4, 2026, Vanda Pharmaceuticals Inc.); the FDA FAERS public dashboard.
What does the label list?
From the label: adverse reactionsThe most common adverse reactions (incidence >5% and at least twice as high on HETLIOZ than on placebo) were headache, increased alanine aminotransferase, nightmares or unusual dreams, and upper respiratory or urinary tract infection ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Vanda Pharmaceuticals Inc.
Table 2: Adverse Reactions in Study 1
| Reaction | 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 % |
Rows and numbers as the label prints them, under its own column headings (the first rows of a longer table where it runs on). A reaction in a trial table is not necessarily caused by the medicine, and trials differ from everyday use.
From the label: adverse reactions, continuedThe most common adverse reactions (incidence >5% and at least twice as high on HETLIOZ than on placebo) were headache, increased alanine aminotransferase, nightmares or unusual dreams, and upper respiratory or urinary tract infection ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Vanda Pharmaceuticals Inc. at 1-844-438-5469 or www.hetlioz.com or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch. 6.1 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. More than 2080 subjects have been treated with at least one dose of HETLIOZ, of which more than 380 have been treated for > 26 weeks and more than 170 have been treated for > 1 year. Non-24-Hour Sleep-Wake Disorder (Non-24) 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. …
Source: FDA drug label, Vanda Pharmaceuticals Inc., effective August 4, 2026 (SPL set ca4a9b63-708e-49e9-8f9b-010625443b90), via openFDA; the full label on DailyMed
What do FAERS reports show?
5,770 reports list tasimelteon among the medicines taken, each a report of something that happened, not proof the medicine caused it; 166 are coded as a death and 287 as a hospital stay. Counts follow how widely a medicine is used and watched, and they are not a measure of how risky it is. Of the 18 reactions below, 4 are named by the same term (in US spelling) in the label's adverse reactions, warnings or patient leaflet; a reaction the label does not name may still be there under another word, and a report does not show that the medicine caused it.
Reports received each year
The 25 most reported reactions
Source: FDA Adverse Event Reporting System (FAERS) via openFDA, reports through July 30, 2026; reactions as MedDRA preferred terms
What next?
- Tasimelteon: the full record
- label, generics, price, shortages, recalls
- Report a side effect to the FDA
- MedWatch
- Side effects hub
- the most reported medicines and reactions