Prenatal Vitamin Side Effects: Nausea, Constipation and Iron Problems
What official sources say about nausea, constipation and other prenatal vitamin side effects, how iron fits in, and which problems to raise with your prenatal care team.
Content last updated . This page has not been medically reviewed.
Urgent first: swallowed vitamins and serious reactions
Prenatal vitamins often contain iron. MedlinePlus says a severe iron overdose can happen if a child eats adult multivitamins, such as prenatal vitamins. [1] If a child may have swallowed some, call Poison Help at 1-800-222-1222 (US). Call 911 if the person has collapsed, had a seizure, has trouble breathing or cannot be awakened. [2] Outside the US, see poison help by country. MedlinePlus says itching, rash or hives after taking a prenatal vitamin are reasons to call your doctor immediately. [2]What the sources list as side effects
MedlinePlus lists constipation, nausea, vomiting and diarrhea as possible side effects of prenatal vitamins, and says to tell your doctor if they are severe or do not go away. [2] That is the whole list the page gives. It does not say how common each one is, which ingredient is responsible, or whether any form of vitamin causes fewer problems.Proper evidence not found in the sources we read (MedlinePlus, NHS, a Cochrane review of iron in pregnancy, PubMed searches) for rates of nausea or constipation by prenatal ingredient or by tablet, capsule or gummy form. For a general look at multivitamin side effects, see multivitamin side effects.
Where iron comes in
Iron is the ingredient people most often ask about when a prenatal vitamin upsets the stomach, and the sources are careful about it.| Question | What the source found | Strength |
|---|---|---|
| What side effects does iron cause at high amounts? | The NHS (UK) says side effects of taking high doses (over 20mg) of iron include constipation, feeling sick, being sick and stomach pain. [3] | Official advice for adults in general, not pregnancy-specific |
| What do people prescribed iron tablets report? | The NHS lists constipation or diarrhoea, tummy pain, heartburn, feeling sick and darker than usual poo, and says to follow the GP's advice about how to take iron tablets. [4] | Official advice |
| Does iron in pregnancy raise side-effect reports in trials? | A 2015 Cochrane review found no clear difference between groups in reporting of side effects (11 trials, 2,423 women), rated very low quality evidence. [5] | Insufficient evidence either way |
In the same review, preventive iron reduced maternal anaemia at term by 70% compared with no iron or placebo. [5] So iron is not a pointless ingredient; whether you need extra is decided by blood tests. In the UK, iron levels are checked at the first midwife appointment and at around 28 weeks. [6] More on products with and without iron: prenatal vitamins without iron, with DHA and other label differences.
Iron amounts from official bodies
| Source and year | Population | Figure |
|---|---|---|
| NHS (UK), page last reviewed 2020 | Adults | Supplements of 17mg or less a day of iron are unlikely to cause harm; continue a higher dose if a GP advises it [3] |
| MedlinePlus (US), page read 2026 | Pregnant adults | 27 mg of iron a day [7] |
| EFSA (EU), overview version 11, August 2025 | Adults, pregnancy and lactation | Safe level of intake 40 mg a day (EFSA could not set a tolerable upper level for iron) [8] |
These are different kinds of figure from different countries, not one rule. Amounts used in studies are not advice for you.
Nausea in early pregnancy has other causes
The NHS says nausea and vomiting in early pregnancy is very common and usually clears up by weeks 16 to 20. [9] So a prenatal vitamin may be blamed for sickness that would happen anyway. The sources we read do not say whether any prenatal vitamin changes that. The NHS suggests small, frequent meals of plain foods and says to talk to a pharmacist before taking ginger supplements in pregnancy. [9] If you cannot keep food or fluids down for 24 hours, the NHS lists that among the reasons to contact your midwife or GP. [9]Safety points
- Vitamin A. The NHS says not to take cod liver oil or supplements containing vitamin A (retinol) in pregnancy. [6] EFSA's overview lists an upper level for preformed vitamin A of 3,000 micrograms RE a day for adults, pregnancy and lactation. [8] Official advice differs; check the label and ask your care team. See vitamin A.
- Medicines and other supplements. Tell your doctor and pharmacist about everything you take, and mention bleeding problems or anemia. [2] See medication interactions.
- Surgery. Tell the surgeon or dentist you take prenatal vitamins. [2]
- Kidney or liver conditions. The prenatal vitamin pages we read do not cover these; ask your clinician.
- Doubling up. Separate iron, calcium or fish-oil products can duplicate what is in a prenatal product. Ask your care team before combining.
- Quality. Labels can be wrong. In a 2025 study of 47 prenatal products, 20 of the 25 that listed iodine contained more than 20% less than the label said. [10] See quality testing.
Do not stop or change a prenatal vitamin on your own because of this page; talk to your midwife, doctor or pharmacist. See also prenatal and postnatal vitamins.
Frequently asked questions
Why do prenatal vitamins make me feel sick?
The sources we read do not say. MedlinePlus lists nausea and vomiting as possible side effects, and the NHS notes that morning sickness is very common in early pregnancy anyway. [2][9] Ask your midwife, doctor or pharmacist.
Can prenatal vitamins cause constipation?
MedlinePlus lists constipation as a possible side effect, and the NHS lists it for high amounts of iron. [2][3] Tell your care team if it is severe or does not go away.
Is the iron in prenatal vitamins the problem?
Iron can cause stomach side effects at high amounts, according to the NHS, but a Cochrane review found no clear difference in side-effect reports between iron and no iron. [3][5] Proper evidence not found that removing iron fixes symptoms.
When should I call someone about vomiting?
The NHS says to contact your midwife or GP if you cannot keep food or fluids down for 24 hours, among other signs. [9] Itching, rash or hives after a vitamin should be reported to your doctor straight away. [2]
What if my child swallowed my prenatal vitamins?
In the US call Poison Help at 1-800-222-1222 right away, and 911 if the child has collapsed, had a seizure, has trouble breathing or cannot be awakened. [2][1]
References
- MedlinePlus Medical Encyclopedia: Iron overdose. Read 2026-10-23.
- MedlinePlus: Prenatal Vitamins (drug information, last revised 15 March 2025). Read 2026-10-23.
- NHS: Vitamins and minerals - Iron (page last reviewed 3 August 2020). Read 2026-10-23.
- NHS: Iron deficiency anaemia. Read 2026-10-23.
- Peña-Rosas JP et al. Daily oral iron supplementation during pregnancy. Cochrane Database Syst Rev 2015 (PubMed 26198451); a 2024 update exists and was not read. Read 2026-10-23.
- NHS: Pregnancy vitamins and supplements (page last reviewed 3 June 2026). Read 2026-10-23.
- MedlinePlus: Pregnancy and Nutrition (health topic). Read 2026-10-23.
- EFSA: Overview on Tolerable Upper Intake Levels as derived by the SCF and the EFSA NDA Panel, version 11, August 2025 (PDF). Read 2026-10-23.
- NHS: Vomiting and morning sickness (page last reviewed 17 April 2024). Read 2026-10-23.
- Content of selected nutrients and heavy metals in prenatal multivitamins and minerals: an observational study. Am J Clin Nutr 2025 (PubMed 40252729). Read 2026-10-23.