Can vitamin A cause constipation?

We found no trial and no systematic review that links vitamin A to constipation. The one gut effect pooled from randomized trials is vomiting. In a Cochrane review of 4 studies in 10,541 children aged 6 months to 5 years, a vitamin A megadose made vomiting in the next 48 hours about twice as likely. The risk ratio was 1.97 (95% CI 1.44 to 2.69), and constipation was not counted. At toxic intakes, mostly from supplements, vitamin A can raise blood calcium, and high blood calcium can slow the bowel. How often that ends in constipation, nobody has measured.

Unsettled. This is an absence of evidence, and the reason matters. The reviews of vitamin A trials we read report harms such as vomiting and bulging fontanelle, and none of them reports bowel habit. Trials of supplements for constipation did not test vitamin A either. So the honest answer for ordinary doses is that we found no one who looked, and for toxic doses a slow bowel is plausible through calcium without a counted case that we could find.

The two pooled gut effects of supplements on this page, as risk ratios

Randomized trials, pooled

The first row is a single high dose of vitamin A in children aged 6 months to 5 years, and constipation was not an outcome. The second row is daily iron in menstruating women and has nothing to do with vitamin A. Neither review gives the share of people affected in each arm in the abstract we read, so only ratios are shown. Sources: cards ev-vac-02 and ev-vac-09 below.

What the trials found

Trials of vitamin A

The largest harms analysis pooled 120 randomized trials with 1,671,672 people, and no trial in it reported serious adverse events from vitamin A. That review did not pool minor gut effects such as constipation, and only 13% of the individually randomized trials were at low risk of bias. A 2024 Cochrane review of oral vitamin A in children up to 7 years found harms checked in one trial of 116 infants. No infant on vitamin A or on placebo had vomiting, feeding difficulties or a bulging fontanelle, on very low certainty evidence.

Trials for constipation

A 2023 meta-analysis of supplements for chronic constipation found 8 randomized trials with 787 adults. They tested kiwifruit, senna, magnesium oxide, jujube and mallow. None tested vitamin A, so there is no trial evidence that vitamin A helps constipation either.

Vitamin A from food

In NHANES survey data on 11,722 US adults, men eating the most lycopene had lower odds of chronic constipation than those eating the least, an odds ratio of 0.55 (95% CI 0.36 to 0.84). Women eating the most alpha-carotene had an odds ratio of 0.69 (95% CI 0.48 to 0.98). Beta-carotene and the other carotenoids showed no link after full adjustment. The design is cross-sectional, carotenoids come with fruit and vegetables and their fiber, and lycopene is not a source of vitamin A at all. What the survey does say is that colorful plant foods were not tied to more constipation.

Vitamin A toxicity in published case reports, percent of reports

These are shares of 31 published reports of vitamin A toxicity, mostly single cases, not shares of people who take vitamin A. The abstract does not say which gut symptoms were seen, so constipation cannot be separated out. Source: card ev-vac-06 below.

Who should be careful

The calcium route is the one plausible link. A 2026 systematic review of 31 published reports of vitamin A toxicity found high blood calcium in 29% of them and gut symptoms in 26%, and supplements were the source in 21 of the 31. Those are shares of case reports, not of people taking vitamin A, and the abstract does not say which gut symptoms. A 2025 case report traced an older woman's high blood calcium to hidden excess vitamin A from supplements. What high blood calcium does to the bowel is clearest in a different vitamin. Among 38 young children with vitamin D overdose, including from faulty fish oil, constipation was among the most common symptoms, in 31.6%. That study is about vitamin D, and no such count exists for vitamin A.

Not for everyone. If you take a high-dose vitamin A product and develop new constipation with nausea, tiredness or aching muscles, the dose is worth raising with a doctor, who may check blood calcium. Women who are or might be pregnant, and those breastfeeding, are advised by NIH not to take more than 3,000 mcg RAE (10,000 IU) of vitamin A from supplements a day, because of the risk of birth defects. People on the weight loss drug orlistat absorb less vitamin A and beta-carotene, and NIH notes this can lower blood levels in some patients.

If a multivitamin seems to constipate you, iron is the better suspect. In a Cochrane review of 8 studies in 1,036 menstruating women, daily iron doubled hard stools or constipation, a risk ratio of 2.07 (95% CI 1.35 to 3.17) on high quality evidence. That review is about iron alone. Whether iron explains constipation on a multivitamin that also contains vitamin A is our inference, and no trial has tested it.

What expert bodies say

NIH lists the signs of acute vitamin A toxicity as severe headache, blurred vision, nausea, dizziness, aching muscles and coordination problems. Regular high doses, it says, cause dry skin, painful muscles and joints, fatigue, depression and abnormal liver tests. Constipation is on neither list. EFSA in 2024 kept the upper limit for preformed vitamin A at 3,000 micrograms RE a day for adults, including pregnant and breastfeeding women. That limit was set to prevent birth defects, and in what we read no expert body names constipation as a harm of vitamin A.

How we searched

Searched: a local copy of PubMed, queried on 7 October 2026 across all study types. A broad query for vitamin A, retinol, beta-carotene or hypervitaminosis with constipation, bowel or gut symptoms returned 2196 hits, mostly off target. Narrower queries returned 134 hits for vitamin A or carotenoids with constipation, 1271 for vitamin A toxicity, 37 for survey data on intake and constipation, and 332 for adverse effects of vitamin A in children. We also searched the NIH Office of Dietary Supplements fact sheet, ClinicalTrials.gov, which had no registered trial, and the web for recent surveys and the EFSA opinion.

Included: five meta-analyses and systematic reviews of randomized trials, one survey of diet, one systematic review of case reports and one case report. Also included are a case series on vitamin D overdose, used only to show how high calcium presents, four sections of the NIH fact sheet and the EFSA upper limit opinion.

Excluded: a trial of a tube feed combining fiber, fish oil and carotenoids, where the carotenoids cannot be separated out. Two reviews of newborn vitamin A that repeat a harm already covered. Studies of retinoid drugs such as isotretinoin, which are medicines rather than vitamin A from food or supplements. Case reports of high calcium from a vitamin D metabolism disorder.

What we read: abstracts for most sources. The NHANES survey and the EFSA opinion were read in full text. All sources with a PubMed record were checked against Retraction Watch, with no retractions.

What we could not get: the full text of the review of 31 toxicity reports, which would say whether its gut symptoms included constipation. A web search pointed to a table in another article reporting constipation on a vitamin A supplement, but the page blocked automated access and we could not identify the trial, so it is not used here.

What would change this answer

A trial, or a pooled harms analysis, that records bowel habit in people taking daily preformed vitamin A at or above the upper limit. We found none, and none is registered.
A case series of vitamin A toxicity that counts constipation separately from nausea and vomiting.
The table we could not open, if it turns out to come from a placebo-controlled trial of vitamin A.

The rest of the nutrient, in one place. Vitamin A: the supplement that increased cancer in a randomized trial → What it does, how much you need, who runs short, and what too much does, with the evidence level shown on every line.

The full guide

Vitamin A: the supplement that increased cancer in a randomized trial

The same question for other foods (constipation)

Sources

ev-vac-01 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials · n = 787
Eight RCTs (787 participants) were included, investigating kiwifruit (n = 3 RCTs), senna (n = 2), magnesium oxide (n = 2), Ziziphus jujuba (n = 1), and Malva Sylvestris (n = 1) supplements.
Who: adults with chronic constipation in RCTs of food, vitamin or mineral supplements
Effect: no vitamin A trials found; magnesium oxide responders RR 3.32 (95% CI 1.59 to 6.92)
Certainty: The absence of any vitamin A trial is the finding for this pair; Mg numbers are context.
Neurogastroenterol Motil, 2023 · checked 2026-10-07 · we read the abstract
ev-vac-02 · Meta-analysis or systematic review · Cochrane systematic review and meta-analysis of randomized controlled trials · n = 10,541
There was an increased risk of vomiting within the first 48 hours of VAS (RR 1.97, 95% CI 1.44 to 2.69; 4 studies, 10,541 children; moderate-certainty evidence).
Who: community-living children aged 6 to 59 months given synthetic vitamin A supplements
Effect: vomiting within 48 hours RR 1.97 (95% CI 1.44 to 2.69), moderate certainty; constipation not reported as an outcome
Certainty: Moderate-certainty GRADE for vomiting. Short-lived effect of a single high dose; no bowel habit outcome.
Cochrane Database Syst Rev, 2022 · checked 2026-10-07 · we read the abstract
ev-vac-03 · Meta-analysis or systematic review · systematic review of randomized trials with meta-analysis and trial sequential analysis · n = 1,671,672
No trial reported serious adverse events or quality of life.
Who: people of any age in trials of vitamin A supplements vs placebo or no intervention
Effect: no serious adverse events reported in any trial; slight increase in bulging fontanelle in neonates and infants
Certainty: Only 13% of individually randomized trials at low risk of bias; GI side effects such as constipation were not pooled.
BMJ Open, 2024 · checked 2026-10-07 · we read the abstract
ev-vac-04 · Meta-analysis or systematic review · Cochrane systematic review and meta-analysis of randomized controlled trials · n = 116
No infant in either the placebo or vitamin A group was found to have feeding difficulties (failure to feed or vomiting), a bulging fontanelle, or neurological signs before or after vitamin A administration (very low-certainty evidence).
Who: children up to seven years in RCTs of oral vitamin A to prevent upper respiratory infections; harms in one study of infants
Effect: no feeding difficulties, vomiting, bulging fontanelle or neurological signs in either arm
Certainty: Very low certainty for adverse events; one small study.
Cochrane Database Syst Rev, 2024 · checked 2026-10-07 · we read the abstract
ev-vac-05 · Observational data · cross-sectional analysis of a national survey · n = 11,722
No association was found between other types of carotenoids and chronic constipation in men and women.
Who: US adults, NHANES 2005 to 2010, constipation by Bristol Stool Form Scale types 1 and 2, two 24-h recalls
Effect: men, lycopene Q4 vs Q1 OR 0.55 (0.36 to 0.84); women, alpha-carotene Q4 vs Q1 OR 0.69 (0.48 to 0.98); no association for other carotenoids
Certainty: Cross-sectional; carotenoids travel with fruit and vegetables (fiber), so this is not a test of vitamin A. Lycopene is not provitamin A. No data on preformed vitamin A or supplements.
BMC Public Health, 2023 · checked 2026-10-07 · we read the abstract
ev-vac-06 · Laboratory or animal data · systematic review of case reports and case series · n = 31 studies
The most common adverse effect was hypercalcemia (n = 9, 29.0%), followed by gastrointestinal manifestations (n = 8, 25.8%), neurological manifestations (n = 7, 22.6%), hepatobiliary manifestations (n = 7, 22.6%), musculoskeletal manifestations (n = 6, 19.4%), and dermatological manifestations (n = 5, 16.1%).
Who: published cases of hypervitaminosis A (26 case reports, 3 case series, 2 observational studies)
Effect: hypercalcemia 9 of 31 (29.0%); gastrointestinal manifestations 8 of 31 (25.8%); supplements the source in 21 of 31 (67.7%)
Certainty: Built on case reports; percentages are of reports, not of people taking vitamin A. GI manifestations are not broken down by type in the abstract (no_pmc).
Clin Chim Acta, 2026 · checked 2026-10-07 · we read the abstract
ev-vac-07 · Laboratory or animal data · case report · n = 1
While it is essential for numerous physiological processes, caution is required when supplementing at high doses, as excessive intake can result in vitamin A toxicity, which may present with hypercalcemia.
Who: one elderly woman taking vitamin A supplements
Effect: hypercalcemia attributed to excessive vitamin A intake
Certainty: Single case; shows the calcium route, not constipation itself.
Cureus, 2025 · checked 2026-10-07 · we read the abstract
ev-vac-08 · Observational data · retrospective case series of records · n = 38
Vomiting (65.8%), loss of appetite (47.4%), and constipation (31.6%) were the most common symptoms.
Who: children aged 0.3 to 4 years with vitamin D intoxication, Turkey, 2015 to 2019
Effect: vomiting 65.8%, loss of appetite 47.4%, constipation 31.6%
Certainty: About vitamin D, not vitamin A. Shown because both raise blood calcium, and this is how high calcium shows up in the gut; no such count exists for vitamin A.
Arch Pediatr, 2021 · checked 2026-10-07 · we read the abstract
ev-vac-09 · Meta-analysis or systematic review · Cochrane systematic review and meta-analysis of randomized controlled trials · n = 1,036
eight studies recruiting 1036 women identified an increased prevalence of hard stools/constipation (RR 2.07, 95% CI 1.35 to 3.17, high quality evidence).
Who: menstruating women in trials of daily oral iron supplementation
Effect: hard stools or constipation RR 2.07 (95% CI 1.35 to 3.17), high quality evidence
Certainty: About iron, not vitamin A; that iron explains constipation on multivitamins with vitamin A is an inference, not tested.
Cochrane Database Syst Rev, 2016 · checked 2026-10-07 · we read the abstract
ev-vac-10 · Position of an expert body · agency fact sheet · n = —
Resulting signs and symptoms typically include severe headache, blurred vision, nausea, dizziness, aching muscles, and coordination problems.
Who: people who take one or a few very high doses of preformed vitamin A
Effect: acute toxicity within days to weeks after doses typically more than 100 times the RDA; constipation not among listed signs
Certainty: Agency summary; absence of constipation from the list is our reading.
NIH Office of Dietary Supplements, Vitamin A and Carotenoids Fact Sheet for Health Professionals · checked 2026-10-07 · we read the section
ev-vac-11 · Position of an expert body · agency fact sheet · n = —
Chronic hypervitaminosis A (regular consumption of high doses) can cause dry skin, painful muscles and joints, fatigue, depression, and abnormal liver test results [85].
Who: people regularly taking high doses of preformed vitamin A
Effect: chronic hypervitaminosis A: dry skin, painful muscles and joints, fatigue, depression, abnormal liver tests
Certainty: Constipation not listed; hypercalcemia route comes from case data (ev-vac-06).
NIH Office of Dietary Supplements, Vitamin A and Carotenoids Fact Sheet for Health Professionals · checked 2026-10-07 · we read the section
ev-vac-12 · Position of an expert body · agency fact sheet · n = —
Experts advise women who are or might be pregnant and those who are lactating not to take high doses (more than 3,000 mcg RAE [10,000 IU] daily) of vitamin A supplements [1].
Who: women who are or might be pregnant and lactating women
Effect: avoid supplemental vitamin A above 3,000 mcg RAE (10,000 IU) a day because of birth defect risk
Certainty: Relevant to anyone taking a high-dose vitamin A product for any reason, including bowel complaints.
NIH Office of Dietary Supplements, Vitamin A and Carotenoids Fact Sheet for Health Professionals · checked 2026-10-07 · we read the section
ev-vac-13 · Position of an expert body · agency fact sheet · n = —
Orlistat (Alli, Xenical), a weight-loss treatment, can decrease the absorption of vitamin A, other fat-soluble vitamins, and beta-carotene, resulting in low plasma levels in some patients [86,87].
Who: people taking orlistat (Alli, Xenical)
Effect: reduced absorption of vitamin A and beta-carotene, low plasma levels in some patients
Certainty: Agency summary of drug interaction.
NIH Office of Dietary Supplements, Vitamin A and Carotenoids Fact Sheet for Health Professionals · checked 2026-10-07 · we read the section
ev-vac-14 · Position of an expert body · scientific opinion of the EFSA NDA Panel with systematic reviews · n = —
The Panel proposes to retain the UL of 3000 μg RE/day for adults, based on a NOAEL for teratogenicity (Section 3.7.1.1). This UL applies to men and women, including women of child‐bearing age, pregnant and lactating women, and post‐menopausal women.
Who: EU population; adults including women of child-bearing age, pregnant and lactating women
Effect: UL 3000 ug RE/day for adults from all sources, based on a NOAEL for teratogenicity; agrees with the US UL of 3,000 mcg RAE
Certainty: Level E. The UL is not based on gut effects; no body names constipation as a vitamin A harm.
EFSA NDA Panel, Scientific opinion on the tolerable upper intake level for preformed vitamin A and beta-carotene, EFSA Journal 2024;22(6):e8814 · checked 2026-10-07 · we read the fulltext

How this page was made. Evidence was extracted from primary sources into cards, every number was re-checked against the source, and the text was written from those cards. Bioma Learn has no human medical reviewer at this time, and we say so rather than invent one. Methodology. This is information, not medical advice.