Collagen side effects: downsides and who should be careful

What are the side effects of collagen?

In clinical trials, collagen supplements have caused few side effects, and those reported were usually mild. The ones people mention most often are:

• Digestive effects – a feeling of fullness or heaviness, bloating, mild stomach upset or heartburn. Taking it with food may be more comfortable.
• Taste and smell – unflavoured collagen has its own mild taste and aroma, and marine collagen can taste fishy.
• Reactions to added ingredients – sweeteners, flavourings or other active ingredients in combination products, rather than the collagen itself.

Most trials ran for weeks or months, not years, so long-term data is limited. That is one more reason not to take more than the amount on the label.

What is the downside of taking collagen?

Apart from mild side effects, the main downsides are practical:

• The evidence is uncertain. Research on skin and joints is mixed, and no health claim for collagen is authorised in the UK – see Do collagen supplements work?.
• It is an incomplete protein. Collagen lacks the essential amino acid tryptophan, so it should not replace meat, fish, eggs, dairy or pulses.
• Cost. It is an ongoing expense for an uncertain result.
• It is not suitable for everyone’s diet. Collagen always comes from animals, so it is never vegetarian or vegan.

Who should avoid taking collagen?

• People with a fish or shellfish allergy should avoid marine collagen, which is made from fish skin and scales, and check the allergen information on any collagen product.
• People with an egg allergy should check the label, as some products use eggshell membrane.
• Vegetarians and vegans – collagen comes from cattle, pigs or fish.
• People who avoid beef or pork for religious or ethical reasons – check the source, and look for halal or kosher certification if that matters to you. Gelatine and porcine collagen come from pigs.
• Children – collagen supplements are not intended for children.

Can I take collagen when pregnant or breastfeeding?

There is very little research on collagen supplements in pregnancy or breastfeeding, so their safety has not been established. The NHS advises taking a folic acid supplement before and in early pregnancy and a daily vitamin D supplement, and getting other nutrients from a healthy, varied diet. Talk to your midwife, GP or pharmacist before taking any other supplement while pregnant or breastfeeding.

Is collagen safe if you have kidney disease?

Collagen is protein, and people with chronic kidney disease are sometimes advised to watch their protein intake. Collagen also contains hydroxyproline, which the body can convert to oxalate – a component of the most common type of kidney stone. If you have kidney disease or a history of kidney stones, speak to your GP or kidney team before taking collagen.

Can I take a collagen supplement with medication?

No significant interactions between pure collagen and common medicines are known, but this has not been studied closely. Combination products are a different matter: many “beauty” collagen products add vitamins, minerals or herbal extracts, and those added ingredients are more likely to matter alongside medicines than the collagen itself.

If you take any regular medicine or have a long-term condition such as kidney or liver disease, tell your GP or pharmacist about every supplement you use, including collagen, before you start.

What should you avoid when taking collagen?

• Taking more than the amount on the label – more is not better, and large amounts are more likely to upset your stomach.
• Using it instead of other protein foods – it does not supply all the essential amino acids.
• Doubling up on the same added ingredients – check that a collagen blend does not repeat vitamins or minerals you already take in another supplement.
• Relying on it for a medical problem – joint pain, osteoarthritis, osteoporosis and skin conditions need proper diagnosis and treatment from your GP.

Is collagen fattening?

Not in itself. Collagen is protein, which provides about 4 kcal per gram. A typical serving of 10–15 g therefore adds around 40–60 kcal – similar to a small piece of fruit. Pure collagen powder has no fat and no sugar, but flavoured powders, drinks and gummies can contain added sugar, so check the label.

Weight changes depend on your overall energy intake and activity, not on any single food. Collagen has not been shown to cause weight loss either.