I have to buy enzymes for my 10 year old Old English Sheepdog, he's been on them for over 4 years and I don't think he's going to stop needing them anytime soon. Having to buy the enzymes every month is expensive but Black Cat are competitively priced (in the past when I've reviewed a product as being 'the cheapest' the seller puts the price up!), so I won't say that! What I will say is that although there is a huge difference between buying online and what my local vet charges, and that most of the online sellers are charging roughly the same. However if I am going to spend my money somewhere (and really I have no choice in that respect), I am going to choose the company that offers the best customer service, super quick (j.i.t) delivery and an complicated, easy to use website - Black Cat Medicines is the best.