Which category of drug was previously used for relaxation but has dangerous side effects leading to its replacement?

Barbiturates are indeed the correct choice in this context, as they were historically used as sedatives and anxiolytics to promote relaxation and relieve anxiety. These drugs act as central nervous system depressants, which can produce calming effects. However, their use has significantly declined due to their dangerous side effects, which include a high potential for addiction, respiratory depression, and overdose. The narrow therapeutic window of barbiturates—meaning the difference between effective and harmful doses is small—led to many accidents and deaths, prompting a shift to safer alternatives.

Benzodiazepines, while also used for relaxation and anxiety, have replaced barbiturates as the primary choice for these indications. Though they also carry risks, they are generally considered to have a wider safety margin compared to barbiturates. SSRIs and typical antipsychotics serve different purposes in psychiatric treatment and do not primarily focus on relaxation or anxiety relief in the same way, making them less relevant to the context of this question.