If A results in B being taken as a preventative measure (to stop Q), and then C occurs as a result of that preventative measure, then the causal graph looks lik

If A results in B being taken as a preventative measure (to stop Q), and then C occurs as a result of that preventative measure, then the causal graph looks like A->B->C. That is, both A and B can be said to cause C, in the standard counterfactual sense of the word "cause": if you hold everything but X constant, and add more X, and more Y occurs, then X causes Y. That doesn't mean you shouldn't do B. If Q is worse than C, it's still a good choice. But it does cause C.