Collagen side effects: what you should know

Short answer: pure hydrolysed collagen has a good safety profile in healthy adults. A systematic review of 11 placebo-controlled trials with 805 participants found no reported adverse events (Choi et al., 2019). Caution makes sense with a fish allergy, a tendency to kidney stones, during pregnancy and with combination products, where the risks come from the added ingredients rather than from collagen itself.

The clearest picture of safety comes from trials where some participants get a placebo. Choi et al. (2019) reviewed 11 such trials with 805 participants, who took 2.5 to 10 g of collagen hydrolysate a day for 8 to 24 weeks, or 3 g of collagen tripeptides. The authors conclude that collagen supplementation is generally safe and that no adverse events were reported.

Two honest caveats: the trials lasted six months at most and mostly involved healthy adults. We know less about long-term use and about people with chronic conditions, which is why the sections below go through the situations where it pays to be careful.

Digestive complaints are usually mild and fade once the body gets used to it. If they persist even at a lower dose, stop taking collagen and talk to a doctor.

Marine collagen is made from fish, and fish collagen is a proven allergen. A Japanese study (Hamada et al., 2001) found that antibodies in five of eight patients with a fish allergy reacted to tuna collagen. The reaction was shared across fish species but did not cross over to mammalian collagen.

In practice: if you are allergic to fish, do not take marine collagen, and discuss other collagen sources with an allergist. A shellfish allergy (prawns, crab) is a different allergy. Watch out, though, for joint products with glucosamine, which is often made from crustacean shells.

Collagen contains the amino acid hydroxyproline, part of which the body converts into oxalate, the main component of the most common type of kidney stone. Knight et al. (2006) found that urinary oxalate rose by 43 % after 30 g of gelatin a day. Single doses of 5 and 10 g produced a measurable change, while 1 and 2 g did not.

One collalloc sachet contains 3.3 g of collagen, which sits between the doses where the study saw no change and where it did. The study does not say that collagen causes stones. If you are prone to oxalate stones, though, discuss it with a urologist and drink plenty of fluids. With chronic kidney disease, where a doctor monitors protein intake, count collagen in, because it is a protein.

Clinical trials of collagen have not included pregnant or breastfeeding women, so safety data for them are missing. That does not mean collagen is harmful, only that nobody has studied it properly. Talk to your doctor before you start, and follow the warning on the label: for combination products with vitamins, herbs or mushrooms, manufacturers often advise against use in pregnancy. Do not give adult supplements to children without medical advice.

Interactions between collagen alone and medication are not well documented. The ingredients manufacturers add to collagen deserve more attention:

• Glucosamine and chondroitin (common in joint products): cases of a raised INR, meaning slower blood clotting, have been reported in people on warfarin (Knudsen and Sokol, 2008).
• Vitamin K affects how warfarin works. If you take warfarin, show your doctor the ingredients of every supplement.
• Biotin (common in hair, skin and nail products) can distort some lab tests, such as troponin, which is measured when a heart attack is suspected (FDA). Mention it before a blood test.
• Polyols (sorbitol, xylitol, maltitol) have a laxative effect in larger amounts. Under EU Regulation 1169/2011, products with more than 10 % added polyols must carry a warning on the pack.

Looking for the side effects of a specific product? Check the ingredients. With plain collagen the list of risks is short; with combinations, the risks of every added substance come on top.

This article is about collagen taken by mouth as a food supplement. Injections are a different category: injectable skin fillers carry a risk of bruising, swelling and infection (Pu et al., 2023), and the risks of joint injections depend on the product. Both are done by a doctor, who is the right person to discuss their risks with.

1. Read the ingredients. The shorter the list, the fewer things can go wrong.
2. Start gradually. With sensitive digestion, take half a dose for the first week.
3. Ask for lab results. For marine collagen, ask about heavy metals and microbiology. A serious manufacturer publishes the results.
4. Talk to a doctor if you have a fish allergy, a tendency to kidney stones, are pregnant, breastfeeding or take medication regularly.

collalloc collagen contains only hydrolysed marine collagen, with no sweeteners, colourings or flavourings. Microbiology is tested for every production batch and heavy metals every month by an external laboratory. You will find the results, including the limits, on the product page and on our Laboratory page. As the collagen comes from fish, it is not suitable if you have a fish allergy.

How to take collagen so that it absorbs well and causes no trouble is covered in How to take collagen correctly.