Vitamin D is a hormone created by the body when skin is exposed to ultraviolet rays in sunlight. It is found naturally in a small number of foods and can also be taken as a fat-soluble dietary supplement.

Safety and precautions

Vitamin D is generally considered safe at moderate levels. 

Risk of cancer

Some increased risks of cancer are seen with higher 25(OH)D levels.

Cancer as a whole

Good evidencesignificant effects in one large or several mid-sized and well-designed clinical studies (randomized controlled trials (RCTs) with an appropriate placebo or other strong comparison control or observational studies that control for confounds) (this is the CancerChoices definition; other researchers and studies may define this differently) of higher cancer mortality among men without cancer at baseline with 25(OH)D levels above 40 ng/mL

Breast cancer

Good evidence of moderately higher risk of invasive breast cancer among postmenopausal women with vitamin D intakes higher than 600 IU/day at baseline treated with calcium plus vitamin D

Colorectal cancer

Good evidence of higher risk of advanced conventional adenomas among older adults treated with vitamin D3 and omega-3 fatty acids

Weak evidenceone or more case studies, supported by animal evidence OR small treatment effects of limited clinical significance OR studies with no controls OR weak trends of effects (this is the CancerChoices definition; other researchers and studies may define this differently) of higher risk of colorectal cancer among women treated with estrogen therapy and also treated with vitamin D3 and calcium

Lung cancer

Modest evidence of higher lung cancer mortality among men without cancer at baseline with 25(OH)D levels higher than 40 ng/mL

Pancreatic cancer

Modest evidencesignificant effects in at least three small but well-designed randomized controlled trials (RCTs), or one or more well-designed, mid-sized clinical studies of reasonably good quality (RCTs or observational studies), or several small studies aggregated into a meta-analysis (this is the CancerChoices definition; other researchers and studies may define this differently) of higher risk of pancreatic cancer among people with high 25(OH)D levels

Prostate cancer

Good evidence of higher risk of prostate cancer among people with higher 25(OH)D levels, especially very high levels

Skin cancer

Modest evidence of higher risks of melanoma and other skin cancers among people with higher 25(OH)D levels or with higher intake of vitamin D

Interactions with other therapies among people with cancer 

  • Vitamin D deficiency reduced effectiveness of the medication rituximab in patients with diffuse large B-cell lymphoma16Bittenbring JT, Neumann F et al. Vitamin D deficiency impairs rituximab-mediated cellular cytotoxicity and outcome of patients with diffuse large B-cell lymphoma treated with but not without rituximab. Journal of Clinical Oncology. 2014 Oct 10;32(29):3242-8.
  • Higher risk of recurrent advanced adenomas and/or 2 or more adenomas after colorectal adenoma surgery among people using aspirin or other nonsteroidal anti-inflammatory drugs (NSAIDs) 4 or more days/week and using 1000 IU/day vitamin D3 compared to those with less frequent NSAID use in a large RCTrandomized controlled trial, a study design in which people are randomly assigned to either an experimental group or a control group to compare the outcomes from different treatments; an RCT is considered a strong design for determining a therapy’s effects17Calderwood AH, Baron JA, et al. No evidence for posttreatment effects of vitamin D and calcium supplementation on risk of colorectal adenomas in a randomized trial. Cancer Prevention Research. 2019 May;12(5):295-304.

Side effects or adverse events

Vitamin D is a fat-soluble vitamin with a risk for toxicity and overdose from taking extremely high doses of supplements. Research has shown that 60,000 IU per day leads to vitamin D toxicity.18Zeratsky K. What is vitamin D toxicity? Should I be worried about taking supplements? Mayo Clinic. Viewed July 13, 2022. ​​Acute toxicity could be caused by doses of vitamin D probably in excess of 10,000 IU/day over long periods.19Marcinowska-Suchowierska E, Kupisz-Urbańska M, Łukaszkiewicz J, Płudowski P, Jones G. Vitamin D toxicity-a clinical perspective. Frontiers in Endocrinology (Lausanne). 2018 Sep 20;9:550; De Vincentis S, Russo A, et al. How much vitamin D is too much? A case report and review of the literature. Endocrine, Metabolic and Immune Disorders Drug Targets. 2021;21(9):1653-1659. We advise medical supervision if you plan to take high doses of supplements (more than 4000 IUs per day). Overdose is unlikely when getting vitamin D from food sources or sun exposure. 

Although once rare, vitamin D toxicity has increasingly been reported in people with serum concentrations ranging between 150 and 1220 ng/mL and serum calcium concentrations between 11.1 and 23.1 mg/dL. Such high levels are typically seen among people taking high doses of vitamin D supplements, with underlying causes including manufacturing errors and/or overdosing by patients or prescribers.20Galior K, Grebe S, Singh R. Development of vitamin D toxicity from overcorrection of vitamin d deficiency: a review of case reports. Nutrients. 2018 Jul 24;10(8):953; Lim K, Thadhani R. Vitamin D toxicity. Brazilian Journal of Nephrology. 2020 Apr 3;42(2):238-244.

Adverse reactions and drug interactions are noted with high 25(OH)D levels, especially at or above 40 ng/mL (100 nmol/L).

  • Vomiting or changes in bowel movements
  • Drowsiness, and in extreme cases, depression, delusions and even coma
  • Calcinosis (the deposit of calcium salts in tissues such as the kidneys, heart, or lungs) and high blood levels of calcium (hypercalcemia);21National Cancer Institute. Vitamin D and Cancer Prevention. October 21, 2013. Viewed November 2, 2021. in severe cases, this can lead to the formation of kidney stones or calcifications in organs
  • Acute renal failure and vitamin A toxicity22Granado-Lorencio F, Rubio E et al. Hypercalcemia, hypervitaminosis A and 3-epi-25-OH-D3 levels after consumption of an “over the counter” vitamin D remedy. A case report. Food and Chemical Toxicology. 2012 Jun;50(6):2106-8.

No adverse effects on reproductive hormone levels among postmenopausal women with stage 1–3a breast cancer treated with 4000 IU compared 600 IU

  • Comparable effects on reproductive hormone levels among postmenopausal women with stage 1–3a breast cancer treated with 4000 IU or 600 IU vitamin D3 daily for 6 months in a mid-sized uncontrolled trial; the researchers concluded that the higher dose did not adversely affect reproductive hormone levels23Shapiro AC, Adlis SA et al. Randomized, blinded trial of vitamin D3 for treating aromatase inhibitor-associated musculoskeletal symptoms (AIMSS). Breast Cancer Research and Treatment. 2016 Feb;155(3):501-12.

Vitamin D in combination with other treatments

Preliminary evidencesignificant effects in small or poorly designed clinical studies OR conflicting results in adequate studies but a preponderance of evidence of an effect (this is the CancerChoices definition; other researchers and studies may define this differently) of higher markers of inflammation, higher fasting blood glucose, and worse 30-second sit to stand testa test of leg strength and endurance as shown by the number of stands a person can complete in 30 seconds performance during participation in progressive resistance training among older adults with overweight or obesity and with type 2 diabetes treated with whey protein and vitamin D3

Do not use (contraindications)

Individuals with disorders of calcium metabolism, gastrointestinal disease, kidney disease, heart disease, or liver disease should consult a physician before using vitamin D supplements.

Helpful links

Wellkasa provides information on interactions among hundreds of natural products, over-the-counter drugs, and prescription medications.

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Nancy Hepp, MS

Lead Researcher and Program Manager
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Ms. Hepp is a researcher and communicator who has been writing and editing educational content on varied health topics for more than 20 years. She serves as lead researcher, program manager, and writer for CancerChoices. Her graduate work in research and cognitive psychology, her master’s degree in instructional design, and her certificate in web design have all guided her in writing and presenting information for a wide variety of audiences and uses. Nancy’s service as faculty development coordinator in the Department of Family Medicine at Wright State University also provided experience in medical research, plus insights into medical education and medical care from the professional’s perspective.

Nancy Hepp, MS Lead Researcher and Program Manager

Dr. Ryan served as a research associate for CancerChoices. She is a licensed and board certified naturopathic physician and acupuncturist in Oregon. Dr. Ryan is the founder of Gentle Natural Wellness, a clinic specializing in bridging classical Chinese medicine with naturopathic medicine to provide individualized, compassionate care for people in the community. A Doctorate of Naturopathic Medicine and Master of Science in Oriental Medicine with honors from the National University of Natural Medicine, research in medical anthropology at the University of Hawai’i and George Mason University, language and culture programs at Obirin University (Tokyo) and Sogang University (Seoul), and studies of Chinese herbal medicine and qigong in China have provided a diverse background that has helped form a foundation for her community health and healing path.

Emily Ryan, ND, MSOM, LAc Research Associate


Laura Pole, RN, MSN, OCNS

Senior Clinical Consultant
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Laura Pole is senior clinical consultant for CancerChoices. Laura is an oncology clinical nurse specialist who has been providing integrative oncology clinical care, navigation, consultation, and education services for over 40 years. She is the co-creator and co-coordinator of the Integrative Oncology Navigation Training at Smith Center for Healing and the Arts in Washington, DC. Laura also manages the “Media Watch Cancer News That You Can Use” listserv for Smith Center/Commonweal. In her role as a palliative care educator and consultant, Laura has served as statewide Respecting Choices Faculty for the Virginia POST (Physician Orders for Scope of Treatment) Collaborative as well as provided statewide professional education on palliative and end-of-life care for the Virginia Association for Hospices and Palliative Care.

For CancerChoices, Laura curates content and research, networks with clinical and organizational partners, brings awareness and education of integrative oncology at professional and patient conferences and programs, and translates research into information relevant to the patient experience as well as clinical practice.

Laura sees her work with CancerChoices as a perfect alignment of all her passions, knowledge and skills in integrative oncology care. She is honored to serve you.

Laura Pole, RN, MSN, OCNS Senior Clinical Consultant

Last update: November 29, 2023

Last full literature review: February 2022

CancerChoices provides information about integrative in cancer care, a patient-centered approach combining the best of conventional care, self care and evidence-informed complementary care in an integrated plan cancer care. We review complementaryin cancer care, complementary care involves the use of therapies intended to enhance or add to standard conventional treatments; examples include supplements, mind-body approaches such as yoga or psychosocial therapy, and acupuncture therapies and self-care lifestyle actions and behaviors that may impact cancer outcomes; examples include eating health-promoting foods, limiting alcohol, increasing physical activity, and managing stress practices to help patients and professionals explore and integrate the best combination of conventionalthe cancer care offered by conventionally trained physicians and most hospitals; examples are chemotherapy, surgery, and radiotherapy and complementary therapies and practices for each person.

Our staff have no financial conflicts of interest to declare. We receive no funds from any manufacturers or retailers gaining financial profit by promoting or discouraging therapies mentioned on this site.