In a letter, is it correct to address a PhD as Dr.? Or should I use Mr. and drop the person's title? For example,
Dr. Jack Frost, PhD
Mr. Jack Frost
The use of the title or honorific Dr.—or the academic abbreviation PhD—with a person's name is considered more polite than simply using Mr.
But, if you choose the polite form, you need to consider what the Oxford's Guide to Canadian English Usage has to say about this. The Guide says "If the degree… is listed after the name, the honorific is not used before the name." The Canadian Style agrees. It is incorrect to write Dr. Jack Frost, PhD.
Also, unless you are an academic, you are more likely to associate the title Dr. with a medical doctor. Depending on the recipient of your letter, address your correspondence as follows:
As a side note, The Canadian Style states that PhD should be written without periods.