To be slanderous or libelous, it must be more than just an opinion and be demonstrated to be untrue. For example, "I ate at Restaurant abc123, and it was the worst food I ever ate !!", is one's personal opinion. No problem. If you say, "I ate at Restaurant abc123, the food was the worst I ever ate and it had rat fur and arsenic in it!", that is materially different. At that point you would be alleging certain FACTS, and unless those facts are proven to be true, you could be subject to claims of libel or slander, depending on whether the communication was verbal or written.