What is evening primrose oil?

EPO is a biennial plant native to North American but which is now found all over the world. The medicinal product is produced from the plant’s seeds. EPO capsules (500–1,300 mg) or oil (150 ml) are available from high-street retailers.

EPO is rich in polyunsaturated omega-6 fatty acids that can help control pain and inflammation. Evidence for the effectiveness of EPO in reducing joint pain in rheumatoid arthritis isn’t conclusive, but there’s some evidence that it can improve morning stiffness. It doesn’t seem to alter long-term disease activity, so you should take it alongside your conventional therapy.

  • Family: Herbal medicine of the Onagraceae family
  • Scientific name: Oenothera biennis
  • Other names: Tree primrose, fever plant, night willowherb, King’s-cure-all, scabish, scurvish, sun drop, suncups

How does it work?

EPO contains 2–15% gamma-linolenic acid (GLA) and 70% linolenic acid (LA, which your body makes into GLA), which are types of polyunsaturated omega-6 essential fatty acids. GLA is important for maintaining a joint’s cell structure and function. Your body makes it into hormone-like substances called prostaglandins, which regulate your immune system and fight joint inflammation. GLA might also suppress inflammatory responses by directly acting on some inflammatory cells.

Several factors can interfere with your body’s production of GLA from LA, including:

  • ageing
  • dietary deficiencies
  • viral infections
  • some diseases.

Sunflower oil and other oils generally used in normal diet contain only LA. EPO is one of the richest sources of pure GLA.

Is it safe?

No recommended safe doses have been found for the use in musculoskeletal conditions, but trials have used doses of 6 g (540 mg GLA) a day. If taken in the correct dose, EPO has no major safety problems.

Common side-effects include:

  • nausea
  • diarrhoea
  • rashes.

If you have epilepsy or seizure disorder you shouldn’t take EPO because it can cause seizures.

Interactions haven’t been well studied, but you should be cautious about using EPO if you take anti-inflammatory drugs (for example cortisone) and anticoagulants because interactions are possible.

Trials for rheumatoid arthritis

Trial 1

In this trial, 49 people with rheumatoid arthritis who were on non-steroidal anti-inflammatory drugs (NSAIDs) were randomised to take one of the following once a day for 12 months:

  • 6 g (540 mg GLA) EPO
  • EPO with fish oil
  • placebo tablets.

Participants were asked to take their normal dose of NSAIDs during the first three months of the trial but were advised to reduce or stop it depending on their symptoms afterwards.

  • 94% of participants who got EPO alone and 93% who received EPO combined with fish oil reported a significant improvement of disease-related symptoms, including pain and morning stiffness, compared to only 30% of the placebo group – this was a significant difference.
  • EPO was also significantly effective in reducing the use of NSAIDs during the trial period.
  • Most participants’ symptoms came back during the three months that followed treatment, so EPO didn’t seem to alter the long-term disease activity.
  • Two participants on EPO withdrew from the trial because of nausea and diarrhoea.

Trial 2

Researchers evaluated the outcome of 40 people with rheumatoid arthritis who received daily doses of either 6 g EPO (540 mg GLA) or olive oil for six months.

  • Participants given EPO had a significant improvement in morning stiffness compared to participants given olive oil, but there were no significant differences between both treatment groups with respect to pain reduction and overall disease severity.
  • Most participants in this trial didn’t stop taking NSAIDs.
  • Four out of 19 participants taking EPO had to withdraw because of nausea, flu-like symptoms or their condition getting worse.