I would not consider you to be high risk for an anaphylactic reaction to milk thistle based on your previous anaphylactic tendencies. First off, the tetanus toxid is designed to stimulate an immune reaction which makes it more likely to have caused an over stimulation of the immune system. The Iodine based dyes contain many other chemicals that are most likely the culprit for the anaphylactic reaction you had. For example, if you eat table salt (most of which all contains iodine) or fish which naturally contains iodine and do not have a reaction to that then you are not truly sensitive to iodine. It is the other chemicals in the dye that caused the problem.
So now let’s discuss milk thistle. This is a naturally occurring herb that has been consumed by humans for thousands of years. It does not have any tendencies to stimulate the immune response and should not contain any synthetic chemicals that might trigger an anaphylactic reaction. I have never heard any accounts of someone having this type of reaction to Milk Thistle. Now on the other side, each person’s body is unique and there is no way to 100% guarantee that you won’t have some type of reaction although I feel this is an extremely unlikely situation.
My advice would be to assume it is safe but take precautions to make sure. For example, the first time you take the product, empty the capsule and rub some of the contents onto your lips. If after 30 minutes you have no itching, swelling or burning take some of the contents of a capsule and rub on the inside of your mouth then rinse out. If you have no reaction to this after 30 minutes then try taking a small amount (maybe 1/4 capsule) mixed with some water. To be safe you can have some Benadryl on hand in case you note any anaphylactic symptoms.
Once you have determined that it is safe for you to take, start working up the full dose over the course of about 1-2 weeks to allow your body to adjust.
Again, I have never seen anyone have an anaphylactic response to milk thistle but you can follow the protocol above just to be safe.