If it's an allergic reaction, you might struggle to get anything done. The problem is that there are many individuals, of many different species have reactions to many different things. If every product on the market that could possibly cause allergic reactions had to carry warnings, then those warnings would have to appear on pretty much everything in the shops.

Having said that, there is nothing to stop you trying to campaign to get something done about it, you are perfectly in your rights to express a grievance and to attempt to gather support and backing for changes to be made.

If you do find enough people seeing the same effects, then there is a chance that you could get the company to print a warning on the packaging. What you have to realise, though, is that the company will perceive that this will negatively impact the sale of their product, so it's a rare company that will do this out a feeling of social responsibility. Most will only do it if there is either a legal requirement (requiring the testing I mentioned previously leading to legislation against the product or ingredients), or, if they feel that the negative publicity generated by campaigners against the product is having a bigger effect than printing the warning would have.

I'm not saying you shouldn't do it, and I really don't think you could get into trouble (except see below). But if you do decide to try, you may find it takes a lot of effort to gain any momentum.

The only way I think you could get into trouble is if you were to present the link between this product and damage to human or animal health as fact. It's best with any campaign material (websites, interviews with media etc) or communications (letters to MP's, the company etc.) to phrase things in such a way that makes it clear that you have a certain amount of anecdotal evidence, though no clear proof, and that you believe the situation warrants either further investigation, or voluntary adoption of packing warnings by the company.