Does Lotronex (alosetron) cause side effects?

Lotronex (alosetron) is a selective serotonin 5-HT3 receptor antagonist used to treat severe diarrhea-predominant irritable bowel syndrome (IBS) among women who have chronic IBS symptoms lasting longer than 6 months and have not responded to conventional IBS treatment.

The discomfort and diarrhea of IBS is believed to be due to abnormal activity of the muscles of the intestines and/or the nerves that control the muscles. One of the chemical messengers which is important in coordinating the activity of intestinal nerves is serotonin. Serotonin and its receptors in the intestines may control pain sensation, contraction of intestinal muscle, and release of fluid into the intestines. 

These actions of serotonin can result in pain and diarrhea. Lotronex, by blocking 5-HT3 receptors, reduces the actions of serotonin and the resulting symptoms. 

Common side effects of Lotronex include:

Serious side effects of Lotronex include:

Drug interactions of Lotronex include the following because these drugs may alter the clearance of Lotronex in the body:

The available data with Lotronex use in pregnant women are insufficient to draw conclusions about any drug-associated risks for major birth defects, miscarriage, or adverse maternal or fetal outcomes. 

It is not known if Lotronex is excreted in breast milk. Lotronex should be used cautiously if breastfeeding.

What are the important side effects of Lotronex (alosetron)?

The most common side effect with alosetron is constipation. One-quarter to one-third of patients may develop this side effect. Severe constipation or intestinal inflammation caused by poor circulation of blood (ischemic colitis) are rare but life-threatening, may require surgery, and may cause death.

Therefore, alosetron must be discontinued immediately, and immediate medical attention should be sought if constipation or the signs of ischemic colitis (rectal bleeding or a sudden worsening of abdominal pain) occur.

Other important but less common side effects include:

Lotronex (alosetron) side effects list for healthcare professionals

The following adverse reactions are described in more detail in other sections of the label:

  • Complications of constipation
  • Ischemic colitis

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 practice.

Patients With Irritable Bowel Syndrome

Table 1 summarizes adverse reactions from 22 repeat-dose studies in patients with IBS who were treated with 1 mg of Lotronex twice daily for 8 to 24 weeks. The adverse reactions in Table 1 were reported in 1% or more of patients who received Lotronex and occurred more frequently on Lotronex than on placebo. A statistically significant difference was observed for constipation in patients treated with Lotronex compared to placebo (p<0.0001).

Table 1. Adverse Reactions Reported in ≥1% of Patients with Irritable Bowel Syndrome and More Frequently on Lotronex 1 mg Twice Daily Than Placebo

Body System
Adverse Reaction
(n = 2,363)
Lotronex 1 mg twice daily
(n = 8,328)
Abdominal discomfort and pain4%7%
Gastrointestinal discomfort and pain3%5%
Abdominal distention1%2%
Regurgitation and reflux2%2%

  • Constipation is a frequent and dose-related side effect of treatment with Lotronex. In clinical studies constipation was reported in approximately 29% of patients with IBS treated with Lotronex 1 mg twice daily (n = 9,316). This effect was statistically significant compared to placebo (p<0.0001).
  • Eleven percent (11%) of patients treated with Lotronex 1 mg twice daily withdrew from the studies due to constipation.
  • Although the number of patients with IBS treated with Lotronex 0.5 mg twice daily is relatively small (n = 243), only 11% of those patients reported constipation and 4% withdrew from clinical studies due to constipation.
  • Among the patients treated with Lotronex 1 mg twice daily who reported constipation, 75% reported a single episode and most reports of constipation (70%) occurred during the first month of treatment, with the median time to first report of constipation onset of 8 days.
  • Occurrences of constipation in clinical trials were generally mild to moderate in intensity, transient in nature, and resolved either spontaneously with continued treatment or with an interruption of treatment.
  • However, serious complications of constipation have been reported in clinical studies and in postmarketing experience. In Studies 1 and 2, 9% of patients treated with Lotronex reported constipation and 4 consecutive days with no bowel movement.
  • Following interruption of treatment, 78% of the affected patients resumed bowel movements within a 2-day period and were able to re-initiate treatment with Lotronex.
  • A similar incidence in elevation of ALT (>2-fold) was seen in patients receiving Lotronex or placebo (1.0% vs. 1.2%).
  • A single case of hepatitis (elevated ALT, AST, alkaline phosphatase, and bilirubin) without jaundice in a patient receiving Lotronex was reported in a 12-week study.
  • A causal association with Lotronex has not been established.
Long-Term Safety
  • Patient experience in controlled clinical trials is insufficient to estimate the incidence of ischemic colitis in patients taking Lotronex for longer than 6 months.

Women With Severe Diarrhea-Predominant Irritable Bowel Syndrome

  • Table 2 summarizes the gastrointestinal adverse reactions from 1 repeat-dose study in female patients with severe diarrhea-predominant IBS who were treated for 12 weeks.
  • The adverse reactions in Table 2 were reported in 3% or more of patients who received Lotronex and occurred more frequently with Lotronex than with placebo.
  • Other events reported in 3% or more of patients who received Lotronex and occurring more frequently with Lotronex than with placebo included upper respiratory tract infection, viral gastroenteritis, muscle spasms, headaches, and fatigue.

Table 2. Gastrointestinal Adverse Reactions Reported in ≥3% of Women with Severe Diarrhea-Predominant Irritable Bowel Syndrome and More Frequently on Lotronex Than Placebo

Adverse ReactionPlacebo
(n = 176)
0.5 mg once daily
(n = 175)
1 mg once daily
(n = 172)
1 mg twice daily
(n = 176)
Abdominal pain3%5%6%7%
Hemorrhoidal hemorrhage2%3%2%2%
Abdominal pain upper1%3%1%1%

  • Adverse reactions reported in another study of 701 women with severe diarrhea-predominant IBS were similar to those shown in Table 2.
  • Gastrointestinal adverse reactions reported in 3% or more of patients who received Lotronex and occurring more frequently with Lotronex than with placebo included constipation (14% and 10% of patients taking Lotronex 1 mg twice daily or 0.5 mg as needed, respectively, compared with 2% taking placebo), abdominal pain, nausea, vomiting, and flatulence.
  • Other events reported in 3% or more of patients who received Lotronex and occurring more frequently with Lotronex than with placebo included nasopharyngitis, sinusitis, upper respiratory tract infection, urinary tract infection, viral gastroenteritis, and cough.
  • Constipation was the most frequent adverse reaction among women with severe diarrhea-predominant IBS represented in Table 2.
  • There was a dose response in the groups treated with Lotronex in the number of patients withdrawn due to constipation (2% on placebo, 5% on 0.5 mg once daily, 8% on 1 mg once daily, and 11% on 1 mg twice daily).
  • Among these patients with severe diarrhea-predominant IBS treated with Lotronex who reported constipation most (75%) reported one episode which occurred within the first 15 days of treatment and persisted for 4 to 5 days.
Other Events Observed During Clinical Evaluation Of Lotronex
  • During its assessment in clinical trials, multiple and single doses of Lotronex were administered, resulting in 11,874 subject exposures in 86 completed clinical studies.
  • The conditions, dosages, and duration of exposure to Lotronex varied between trials, and the studies included healthy male and female volunteers as well as male and female patients with IBS and other indications.
  • In the listing that follows, reported adverse reactions were classified using a standardized coding dictionary.
  • Only those events that an investigator believed were possibly related to Lotronex, occurred in at least 2 patients, and occurred at a greater frequency during treatment with Lotronex than during placebo administration are presented.
  • Serious adverse reactions occurring in at least 1 patient for whom an investigator believed there was reasonable possibility that the event was related to treatment with Lotronex and occurring at a greater frequency in patients treated with Lotronex than placebo-treated patients are also presented.
  • In the following listing, events are categorized by body system. Within each body system, events are presented in descending order of frequency.
  • The following definitions are used: infrequent adverse reactions are those occurring on one or more occasion in 1/100 to 1/1,000 patients; rare adverse reactions are those occurring on one or more occasion in fewer than 1/1,000 patients.
  • Although the events reported occurred during treatment with Lotronex, they were not necessarily caused by it.

Blood and Lymphatic 

Rare: Quantitative red cell or hemoglobin defects, and hemorrhage.


Infrequent: Tachyarrhythmias.

Rare: Arrhythmias, increased blood pressure, and extrasystoles.

Drug Interaction, Overdose, and Trauma

Rare: Contusions and hematomas.

Ear, Nose, and Throat 

Rare: Ear, nose, and throat infections; viral ear, nose, and throat infections; and laryngitis.

Endocrine and Metabolic

Rare: Disorders of calcium and phosphate metabolism, hyperglycemia, hypothalamus/pituitary hypofunction, hypoglycemia, and fluid disturbances.


Rare: Light sensitivity of eyes.


Infrequent: Hyposalivation, dyspeptic symptoms, gastrointestinal spasms, ischemic colitis, and gastrointestinal lesions.

Rare: Abnormal tenderness, colitis, gastrointestinal signs and symptoms, proctitis, diverticulitis, positive fecal occult blood, hyperacidity, decreased gastrointestinal motility and ileus, gastrointestinal obstructions, oral symptoms, gastrointestinal intussusception, gastritis, gastroduodenitis, gastroenteritis, and ulcerative colitis.

Hepatobiliary Tract and Pancreas

Rare: Abnormal bilirubin levels and cholecystitis.

Lower Respiratory

Infrequent: Breathing disorders.


Rare: Muscle pain; muscle stiffness, tightness and rigidity; and bone and skeletal pain.


Infrequent: Hypnagogic effects.

Rare: Memory effects, tremors, dreams, cognitive function disorders, disturbances of sense of taste, disorders of equilibrium, confusion, sedation, and hypoesthesia.

Non-Site Specific 

Infrequent: Malaise and fatigue, cramps, pain, temperature regulation disturbances. Rare: Burning sensations, hot and cold sensations, cold sensations, and fungal infections.


Infrequent: Anxiety.

Rare: Depressive moods.


Rare: Sexual function disorders, female reproductive tract bleeding and hemorrhage, reproductive infections, and fungal reproductive infections.


Infrequent: Sweating and urticaria.

Rare: Hair loss and alopecia; acne and folliculitis; disorders of sweat and sebum; allergic skin reaction; eczema; skin infections; dermatitis and dermatosis; and nail disorders.


Infrequent: Urinary frequency.

Rare: Bladder inflammation; polyuria and diuresis; and urinary tract hemorrhage.

Postmarketing Experience

In addition to events reported in clinical trials, the following events have been identified during use of Lotronex in clinical practice. Because they were reported voluntarily from a population of unknown size, estimates of frequency cannot be made. These events have been chosen for inclusion due to a combination of their seriousness, frequency of reporting, or potential causal connection to Lotronex.


Impaction, perforation, ulceration, small bowel mesenteric ischemia.





What drugs interact with Lotronex (alosetron)?

In vivo data suggest that alosetron is primarily metabolized by cytochrome P450 (CYP) 1A2, with minor contributions from CYP3A4 and CYP2C9. Therefore, inducers or inhibitors of these enzymes may change the clearance of alosetron.

CYP1A2 Inhibitors

  • Fluvoxamine is a known strong inhibitor of CYP1A2 and also inhibits CYP3A4, CYP2C9, and CYP2C19.
  • In a pharmacokinetic study, 40 healthy female subjects received fluvoxamine in escalating doses from 50 to 200 mg/day for 16 days, with coadministration of alosetron 1 mg on the last day.
  • Fluvoxamine increased mean alosetron plasma concentrations (AUC) approximately 6-fold and prolonged the half-life by approximately 3-fold.
  • Concomitant administration of alosetron and fluvoxamine is contraindicated. Concomitant administration of alosetron and moderate CYP1A2 inhibitors, including quinolone antibiotics and cimetidine, has not been evaluated, but should be avoided unless clinically necessary because of similar potential drug interactions.

CYP3A4 Inhibitors

  • Ketoconazole is a known strong inhibitor of CYP3A4.
  • In a pharmacokinetic study, 38 healthy female subjects received ketoconazole 200 mg twice daily for 7 days, with coadministration of alosetron 1 mg on the last day.
  • Ketoconazole increased mean alosetron plasma concentrations (AUC) by 29%.
  • Caution should be used when alosetron and ketoconazole are administered concomitantly.
  • Coadministration of alosetron and strong CYP3A4 inhibitors such as clarithromycin, telithromycin, protease inhibitors, voriconazole, and itraconazole has not been evaluated but should be undertaken with caution because of similar potential drug interactions.
  • The effect of induction or inhibition of other pathways on exposure to alosetron and its metabolites is not known.

Other CYP Enzymes

  • In vitro human liver microsome studies and an in vivo metabolic probe study demonstrated that alosetron did not inhibit CYP enzymes 3A4, 2C9, or 2C19.
  • In vitro at total drug concentrations 27-fold higher than peak plasma concentrations observed with the 1 mg dose, alosetron inhibited CYP enzymes 1A2 (60%) and 2E1 (50%).
  • In an in vivo metabolic probe study, alosetron did not inhibit CYP2E1 but did produce 30% inhibition of both CYP1A2 and N-acetyltransferase.
  • Although not studied with alosetron, inhibition of N-acetyltransferase may have clinically relevant consequences for drugs such as isoniazid, procainamide, and hydralazine.
  • The effect on CYP1A2 was explored further in a clinical interaction study with theophylline and no effect on metabolism was observed.
  • Another study showed that alosetron had no clinically significant effect on plasma concentrations of the oral contraceptive agents ethinyl estradiol and levonorgestrel (CYP3A4 substrates).
  • A clinical interaction study was also conducted with alosetron and the CYP3A4 substrate cisapride. No significant effects on cisapride metabolism or QT interval were noted.
  • The effects of alosetron on monoamine oxidases and on intestinal first pass secondary to high intraluminal concentrations have not been examined.
  • Based on the above data from in vitro and in vivo studies, it is unlikely that alosetron will inhibit the hepatic metabolic clearance of drugs metabolized by the CYP enzymes 2C9, 2C19, or 2E1.
  • Alosetron does not appear to induce the major cytochrome P450 drug-metabolizing enzyme 3A. Alosetron also does not appear to induce CYP enzymes 2E1 or 2C19. It is not known whether alosetron might induce other enzymes.

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Medically Reviewed on 5/27/2020
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Professional side effects and drug interactions sections courtesy of the U.S. Food and Drug Administration.