State Sen. Charles E. Shannon, D-Somerville, was admitted to Massachusetts General Hospital his office announced today.

The senator experienced an adverse reaction to the series of anti-rejection drugs prescribed as follow-up from a bone-marrow transplant. Reactions can cause or accelerate infection, fever, lethargy, internal fluid build-up, or a number of other non-cancer complications, said Sean J. Fitzgerald, the senator’s chief of staff.

After Shannon was diagnosed in December with leukemia, he successfully completed a nine-day chemotherapy program at MGH and was discharged in January, he said.

In February, Shannon underwent a bone-marrow transplant, conducted at Dana-Farber. He returned to work part-time in March, while still maintaining regular outpatient appointments for monitoring and receiving platelets infusions, designed to gradually amass strength, he said.

Shannon is expected to remain at MGH for at least another week, Fitzgerald said.