What is the ruling on the oath: "You are forbidden to me if another man gives you an injection," which the husband said to his wife, knowing that he does not remember his intention when he said it, and that he now denies intending divorce or ẓihār by it? Does ẓihār occur if a man administers an intravenous drip to his wife?

What you have mentioned indicates that you conditioned the prohibition of your wife on her being injected by another man. If what you conditioned it upon occurs, then the reference in this matter is to your intention. If you intended divorce, it becomes a divorce. If you intended zhihar, it becomes zhihar. If you intended an oath by Allah Almighty, or if you did not intend anything, then an expiation for an oath (kaffarat yamin) becomes binding upon you. If you predominantly believe that you did not intend divorce or zhihar, then an expiation for an oath becomes binding upon you.