Dr. Summers had been widely seen as a president who would serve 10, maybe 20, years and usher in landmark changes.
Dr. Summers had also been expected to lead a $5 billion fund-raising campaign that is in the planning stages.
The group sent a similar letter to Dr. Summers last year.
Dr. Summers, through an aide, declined a request for an interview.
Dr. Summers, who said he had used the drug on his 93-year-old grandfather, reported significant success.
Dr. Summers said there was precedent for making such assessments by telephone.
Dr. Summers said that the numbers at most other selective private colleges were similar.
Dr. Summers had a base salary of $563,000 in the 2004-5 academic year, a university spokesman said.
Dr. Summers has made the sciences a major focus of his administration.
Dr. Summers did not respond to several requests for an interview.