Collagen

What are the side effects of collagen?

Updated 9 Nov 2026

Short answer

In trials, collagen caused no more side effects than control. A 2024 review of 35 trials found no rise in side effects or dropouts.[1] People with a fish allergy may react to fish collagen, and large doses of gelatin raised the oxalate in urine.[2][3]

Side effects in trials

A 2024 review of 35 trials in osteoarthritis found that collagen products did not raise the risk of side effects or of dropping out of a trial, compared with control.[1] A 2018 review of 20 supplements for osteoarthritis found no difference in safety between collagen and placebo.[4]

A 2026 review of 25 skin trials found that hydrolyzed collagen was generally well tolerated, with few side effects.[5] In a trial of undenatured type II collagen for 180 days, side effects did not differ from placebo.[6]

Allergy

Collagen supplements are made from cattle, pigs, fish or chicken.[7][6] Fish collagen can cause allergy. In a study of 101 people with fish allergy, 21 (21%) had allergy antibodies, called IgE, to fish collagen. In lab tests, collagen activated allergy cells.[2] The authors say that standard fish allergy tests contain little collagen, so they may miss this allergy.[2]

Kidney stones

Collagen contains hydroxyproline. The body can turn hydroxyproline into oxalate, which forms calcium oxalate kidney stones.[3] On a controlled diet, 30 g of gelatin raised oxalate in urine by 43% compared with 30 g of whey protein. Doses of 5 and 10 g also changed it; doses of 1 and 2 g did not.[3] In another study, 5 healthy men took 30 g of gelatin a day for 3 days, and oxalate in their urine did not change.[8] Neither study counted kidney stones themselves.[3][8]

Protein balance

Collagen lacks tryptophan, an essential amino acid, so it is an incomplete protein.[9] A 2019 calculation found that collagen peptides can replace up to 36% of the protein in a typical US diet. The diet then still gives enough of every essential amino acid.[9]

Pregnancy and children

The skin trials in a 2021 review enrolled adults aged 20 to 70.[10] A 2024 review of training trials included only healthy adults older than 17.[11] No trial of collagen supplements in pregnancy or breastfeeding was found in PubMed.[10]

Who should ask first

Ask a doctor or pharmacist before you take collagen if you have a fish allergy, have had kidney stones, or are pregnant or breastfeeding.[2][3]

Sources (11)

  1. Efficacy and safety of collagen derivatives for osteoarthritis: A trial sequential meta-analysis. Liang CW, Cheng HY, Lee YH, Liao CD, Huang SW Osteoarthritis Cartilage, 2024. PMID 38218227
  2. Collagen-An Important Fish Allergen for Improved Diagnosis. Kalic T, Kamath SD, Ruethers T, Taki AC, Nugraha R, Le TTK, et al. J Allergy Clin Immunol Pract, 2020. PMID 32389794
  3. Hydroxyproline ingestion and urinary oxalate and glycolate excretion. Knight J, Jiang J, Assimos DG, Holmes RP Kidney Int, 2006. PMID 17021603
  4. Dietary supplements for treating osteoarthritis: a systematic review and meta-analysis. Liu X, Machado GC, Eyles JP, Ravi V, Hunter DJ Br J Sports Med, 2018. PMID 29018060
  5. Efficacy and safety of hydrolyzed collagen supplementation on skin health outcomes: a systematic literature review of randomized controlled trials. Bassila C, Bassila JC, Slim M Eur J Clin Nutr, 2026. PMID 42342959
  6. Efficacy and tolerability of an undenatured type II collagen supplement in modulating knee osteoarthritis symptoms: a multicenter randomized, double-blind, placebo-controlled study. Lugo JP, Saiyed ZM, Lane NE Nutr J, 2016. PMID 26822714
  7. Hydrolyzed Collagen-Sources and Applications. León-López A, Morales-Peñaloza A, Martínez-Juárez VM, Vargas-Torres A, Zeugolis DI, Aguirre-Álvarez G Molecules, 2019. PMID 31703345
  8. Theoretical and laboratory investigations of the effects of hydroxyproline ingestion on the metabolic and physicochemical risk factors for calcium oxalate kidney stone formation in a small group of healthy subjects. Rodgers A, Cele P, Ravenscroft N, Edmonds-Smith C, Jackson G Int Urol Nephrol, 2019. PMID 31161522
  9. Significant Amounts of Functional Collagen Peptides Can Be Incorporated in the Diet While Maintaining Indispensable Amino Acid Balance. Paul C, Leser S, Oesser S Nutrients, 2019. PMID 31096622
  10. Effects of hydrolyzed collagen supplementation on skin aging: a systematic review and meta-analysis. de Miranda RB, Weimer P, Rossi RC Int J Dermatol, 2021. PMID 33742704
  11. Impact of Collagen Peptide Supplementation in Combination with Long-Term Physical Training on Strength, Musculotendinous Remodeling, Functional Recovery, and Body Composition in Healthy Adults: A Systematic Review with Meta-analysis. Bischof K, Moitzi AM, Stafilidis S, König D Sports Med, 2024. PMID 39060741

More about collagen

What is collagen?Can you get collagen from food?What is collagen used for?How long does collagen take to work?Is collagen safe?Does collagen help joint pain?

For education; not medical advice. Talk to a doctor before you start, stop or change a supplement.