She was sceptical about the safety of the new drug.

Sceptical : not easily convinced; having doubts or reservations.Sanguine : optimistic or positive, especially in an apparently bad or difficult situation.Hopeful : feeling or inspiring optimism about a future event.Certain : able to be firmly relied on to happen or be the case.Doubtful : feeling uncertain about something.Antonym of Sceptical is Certain.