When are placebo-controlled trials ethically justifiable?

When are placebo-controlled trials ethically justifiable? According to the consensus guideline for ensuring that placebo-controlled trials are conducted ethically, placebo-controlled trials can be justifiable:

Alternatives to placebo-controlled trials: There are many trial design options that can avoid the use of control groups given only inactive placebo. One potential solution is to conduct “add-on” trials (where participants take a standard treatment plus either a new therapy or placebo version of the experimental therapy). Alternatively, “superiority” trials (where participants receive either a standard therapy or the new therapy) can be considered.