Unfortunately, as the seller or distributor of the product, you are liable for injuries caused by the product.  This area of law is called products liability and anyone who had any role in the product whether it is manufacture, sale, distribution, etc. is liable for injuries caused by the product.  As for the warning that the product is only for external use and not to drink it, this provides you with a defense in this potential lawsuit of assumption of the risk.  Assumption of the risk in this case means that the person who drank the product recognized and understood the danger of drinking it and chose to do so anyway.