Benefits
Bitter candytuft extract on its own versus placebo
In the one placebo-controlled trial that gave a bitter candytuft mono-extract by itself, 208 adults with IBS were split among four groups for 4 weeks. The extract alone did not differ significantly from placebo on abdominal pain or the overall symptom score, while two multi-herb blends that contain it did beat placebo.
Lead herb of the nine-herb blend STW 5
In an 8-week placebo-controlled trial in 315 adults with functional dyspepsia, 20 drops of the nine-herb blend STW 5 three times daily improved a symptom score by 6.9 points versus 5.9 with placebo, a small but significant difference. Tolerability matched placebo. This is the whole blend, and nothing here is credited to bitter candytuft alone.
Pooled trials of the nine-herb blend
A Cochrane review pooled 5 trials (814 participants with functional dyspepsia) and found the nine-herb blend STW 5 may improve overall symptoms more than placebo, but judged that evidence very uncertain. Side effects were about as common as with placebo. An earlier pooled analysis found its trials partly through the maker.
What removing bitter candytuft from the blend changed
In a 60-person trial in functional dyspepsia, the nine-herb blend and a version without bitter candytuft both beat placebo over 4 weeks, with no significant difference between them in the end. Relief came somewhat sooner with the full blend. This is a small German-language trial and does not isolate the herb's own effect.
Stomach muscle activity after a single blend dose
In healthy men, one 1.1 mL dose of the nine-herb blend relaxed the upper stomach and increased contractions in the lower stomach compared with control, without changing how fast solid food emptied. In a 103-person dyspepsia trial the blend did not speed stomach emptying either. Neither study tested bitter candytuft alone.
Gut muscle tone in laboratory tissue
Lab work: in guinea pig gut tissue, the bitter candytuft extract did not relax the muscle as peppermint and chamomile extracts did; it raised the resting tone and contraction of slack segments instead. In receptor tests it bound muscarinic M3 receptors, which help control gut contraction. None of this has been measured in people.
Mechanism of action
Raises tone in slack gut muscle (lab work)
In guinea pig intestine, the Iberis amara extract increased the resting tone of relaxed gut segments rather than relaxing them, and in receptor-binding tests it selectively bound muscarinic M3 receptors. The researchers proposed that this tonic action, alongside the relaxing herbs in the blend, gives it a dual action.
Cucurbitacins and glucosinolates
The plant contains cucurbitacins, highly oxygenated triterpenoids that deter insects from eating it, plus glucosinolates (mustard oil glycosides), flavonoid glycosides, kaempferol and sinapic acid. How much of each reaches the body from a liquid extract has not been reported in people.
Inflammation signals pointing both ways in lab studies
In rats, an Iberis amara extract reduced paw swelling and lowered TNF-alpha, interleukin 1-beta and prostaglandin E2. In human esophageal cells, however, the extract alone strongly raised the inflammation signal interleukin 8, which the full nine-herb blend did not. Both studies had co-authors from the blend's maker.
Clinical trials
Double-blind, randomized, placebo-controlled multicenter trial comparing the nine-herb blend STW 5, the six-herb research blend STW 5-II and a bitter candytuft mono-extract with placebo; main outcomes were changes in total abdominal pain and IBS symptom scores at 4 weeks. (Madisch et al. 2004, Aliment Pharmacol Ther)
208 adults with IBS (51 STW 5, 52 STW 5-II, 53 bitter candytuft extract, 52 placebo); 203 completed.
The bitter candytuft extract on its own did not differ significantly from placebo on abdominal pain (P = 0.1473) or the IBS symptom score (P = 0.1207). Both multi-herb blends were significantly better than placebo on both scores (P = 0.0003 to 0.001).
Multicenter, placebo-controlled, double-blind trial of the nine-herb liquid blend STW 5 (Iberogast), 20 drops three times daily for 8 weeks, after endoscopy and a 7-day run-in; primary outcome was change in the Gastrointestinal Symptom Score. (von Arnim et al. 2007, Am J Gastroenterol)
315 adults with functional dyspepsia (158 STW 5, 157 placebo; 308 in the intention-to-treat analysis).
The symptom score improved by 6.9 points with STW 5 and 5.9 points with placebo (P < 0.05), a 1-point difference on top of a large placebo response. H. pylori status did not change the result, and tolerability was similar. The trial tested the whole blend; nothing can be credited to bitter candytuft alone.
Randomized, double-blind, placebo-controlled multicenter trial (German language) comparing the nine-herb blend STW 5, a version without bitter candytuft (STW-5-S) and placebo for 4 weeks. (Madisch et al. 2001, Z Gastroenterol)
60 adults with functional dyspepsia; all completed.
Both blends improved the symptom score more than placebo at 2 and 4 weeks (P < 0.001). The blend without bitter candytuft did as well as the full blend overall (no significant difference), though relief came earlier with the full blend (P = 0.023). A small trial that compares blends rather than testing the herb alone.
Multicenter, placebo-controlled, double-blind trial of the nine-herb blend STW 5 or liquid placebo for 28 days, with a 13C octanoic acid breath test for stomach emptying. (Braden et al. 2009, Neurogastroenterol Motil)
103 adults with functional dyspepsia, about half of them with slow stomach emptying.
The primary outcome, symptom score improvement, did not differ significantly from placebo (P = 0.08), though more people responded to STW 5 (P = 0.03). Stomach emptying did not speed up with STW 5. Bitter candytuft was not tested on its own.
Cochrane systematic review of randomized trials of non-Chinese herbal medicines for functional dyspepsia, with GRADE certainty ratings. (Báez et al. 2023, Cochrane Database Syst Rev)
41 trials with 4477 participants testing 27 herbal medicines; for STW 5 versus placebo, 5 trials with 814 participants for global symptoms.
STW 5 may improve global symptoms versus placebo at 28 to 56 days (mean difference -2.64), but the authors called the evidence very uncertain (very low certainty), with large differences between trials (I2 = 87%). Adverse events showed little to no difference from placebo (risk ratio 0.92). No result concerns bitter candytuft alone.
Laboratory study of the STW 5 blend and its single extracts on isolated guinea pig ileum, with and without acetylcholine or histamine stimulation. (Ammon et al. 2006, Phytomedicine)
Isolated guinea pig intestinal tissue; no human participants.
The blend and its peppermint, chamomile and licorice extracts relaxed induced contractions, but the Iberis amara extract showed no relaxing action and raised the resting tone and contraction of slack segments. Tissue results do not show effects in people.