Как работают антибиотики: когда они эффективны и бесполезны
- Shouldn't "и" be "или" instead? I would think "effective, or useless". Certainly a drug is not both "effective and useless".
That's how I read this title:
Как работают антибиотики: когда они эффективны и (когда они) бесполезны
The 2nd когда они is not writen, but implied. Here the author intends to talk about different situations: those when the medicine is working and those when it is useless. И combines multiple situations (maybe different diseases) into the scope of the article. It cannot be replaced by или.
However, if the author would want to go on per-disease basis or single out a particular kind of illness you could use или there, e.g. "Antibiotics for Flu: Effective or Not?":
Антибиотики при гриппе: эффективны или бесполезны?
- If you were writing this headline, would you write it the same way? Is this very clear and correct usage of the language in your opinion?
You are right though, that the original wording still has a chance to be be misconstrued. If one has to reduce that chance with minimum correction, there is an option to put it like this
Как работают антибиотики: когда они эффективны, а когда бесполезны?