Is it safe to take a 10mg dose of Ativan (lorazepam) all at once.

No. The usual daily dose range of lorazepam (Ativan), a benzodiazepine, is 2 to 6 mg/day. And this would be divided into 1, 2 or 3 mg per dose. As I hope you know, this medication is highly addictive. I am worried that you if you are wanting to take such a large dose that dependence has developed. You need to review your usage with the prescribing doctor and conform to the prescribed plan. Good wishes.