Read our Privacy noticeA warning has been issued over so-called “high strength” omega-3 supplements. The claim of high strength could be leaving shoppers paying a premium for products that contain “surprisingly low” levels of active ingredients compared to cheaper versions, according to consumer group Which? open image in gallery Vitamin C tablets and Omega 3 fish oil liquid capsules ( PA )Which? A Holland & Barrett spokeswoman said: “There is currently no legal or industry-wide definition of what constitutes a ‘high strength’ omega-3 fish oil supplement. “At Holland & Barrett, the term is used to help customers distinguish between the different strengths and formulations of omega-3 products within our range.