In keeping with Tukey's comment, tests of the null hypothesis that two population means or proportions are equal are almost always two-sided (or two-tailed*).
Is the pizza you ordered from the pizza service well done (it couldbe raw, or burnt, or well done) [two sided test]
Generally speaking, we base the decision on the value of a teststatistic, e.g.
Two sided hypothesis test calculator
If a one-sided analysis had been proposed for the trial, these results would have been ignored on the grounds that they were the result of unlikely random variability under a hypothesis of no difference between beta-carotene and placebo.