The problem is that no amount of anecdotal evidence will be taken as proof. You could line up a hundred people who will honestly say 'My cat was OK, I started using this product and within x amount of time he was sick', and the company will respond by lining up a (genuine) 100 other people who say 'I've been using the product for three time as long, and my cats have no trouble'.

That's what scientific research is for, to settle such arguments. To determine if the product is harmful to cats would require vivisection. Cats would have to be exposed to the active ingredients in a controlled laboratory conditions, and the experiment would have to be run over several years with many animals in order to constitute scientific proof. The very great likelihood is that not all cats would respond badly to all levels of exposure to the product. Enough research would have to be conducted to establish the percentage probability of the average cat have a response that was of a level as perceived to be harmful. This would take years, many thousands of pounds (even millions), and would cause suffering in a lot of cats taking part in the experiment.

I'm not saying I like that or agree with it, but thats the way science works, and companies (any company or any product) are only obliged to conform to dangers that have been scientifically established.

The reality is that in this kind of product, there is rarely any substance that cannot be encountered regularly in normal day-to-day life. Thats why every product doesn't have to go through these types of tests: probably all of the ingredients have been tested previously, or are common or garden substances (such as flower essences etc).

The only other alternative is campaigning. If you can muster enough popular support, provide enough anecdotal cases (any case not happening in a laboratory is counted as anecdotal, by the way), get the media involved and kick up a fuss, you may be able to get the company to voluntarily withdraw their product with no scientific evidence, but it is very unlikely.

The best thing to do is to contact the customer services department of the company, and ask if they will give you a complete list of the substances in the product, or at least the 'active ingredients'. If you do this in a calm and reasoned manner, and explain you are just trying to track down possible allergens then I'm sure they will be cooperative.

Once you have that list, you can show it to a vet and you can see if they think there is anything that might cause such a reaction.