Although the majority of reviews on TrustPilot are positive, many of those could be fakes. I'd be more likely to believe the 1 in 6 reviews that say that the product is a total waste of money:
https://uk.trustpilot.com/review/www.viviscal.co.uk
The review on the Holland & Barrett website seem generally good though:
https://www.hollandandbarrett.com/shop/product/viviscal-hair-growth-programme-tablets-60083318
(Scroll down and click 'Load more')
Although the manufacturers claim that their product has been 'shown to work in clinical trials', I note that all of those trials have been conducted by the same doctor (who's a skin care specialist, not a trichologist), with small samples and with (as far as I can tell) no control groups and no double-blind testing. Such 'trials' almost certainly wouldn't be regarded as valid by the scientific community as a whole.
https://www.viviscal.co.uk/the-science