
Kennedy's back problems likely began in 1940 in what has been described as a "minor sports injury." However this back problem was severe enough to prevent Kennedy from joining the US Army in 1941. In September 1941, just two months before his 24th birthday, Kennedy enlisted in the U.S. Navy, getting around any possible medical disqualification with the help of the director of the Office of Naval Intelligence, a man who had formerly served as naval attaché to Ambassador Joseph P. Kennedy. Kennedy was promoted to the rank of lieutenant in 1943 and given command of a Patrol Torpedo (PT) Boat PT-109. On August 2, 1943, the PT-109 was rammed by the Japanese destroyer Amagiri. Kennedy gathered his surviving crew members together in the water around the wreckage and the men swam towards a small island. Kennedy re-injured his back in the collision, but in spite of this, he towed a badly burned crewman through the water with a life jacket strap clenched between his teeth, swimming first to one island, and later to a second island, from where his crew was subsequently rescued. Kennedy returned to the United States in early January 1944 where he received treatment for his back injury. A year later, in January 1945, Kennedy spent another three more months recovering from his back injury at Castle Hot Springs, a resort and temporary military hospital in Arizona. He was honorably discharged just prior to Japan's surrender in 1945.
In November of 1946, Kennedy was elected to Congress as the Representative for Massachusetts 11th Congressional District. But less than a year later, in September of 1947, he was diagnosed by Sir Daniel Davis at The London Clinic with Addison's disease, a rare endocrine disorder in which the adrenal glands do not produce sufficient steroid hormones. According to a report published in the Annals of Internal Medicine in 2009, the cause was most likely caused a rare autoimmune disease called Autoimmune Polyendocrine Syndrome Type 2, or APS 2, which also caused Kennedy's hypothyroidism (a condition in which his thyroid gland produced insufficient hormones).
Kennedy's condition was not made public, but reports of it surfaced in the press when Kennedy was running for president in 1960. At the time Kennedy obfuscated the issue by denying that he had Addison's Disease "from tuberculosis." His physician, Dr. Janet G. Travell told the media that the PT boat incident and possibly malaria caused a depletion of adrenal function "from which he is now rehabilitated." What she left out was that he was still being treated for the condition.
Kennedy's back pain persisted after the war and caused him considerable distress throughout his presidency. In 1954, following one of these surgeries, Kennedy went into a post-operative coma and contracted septicemia. Doctors were concerned that he would not recover and Kennedy received the sacrament of last rites from a Catholic priest. He had a total of five back surgeries. His final surgical procedure was in September 1957 and it was said to have left him disenchanted with finding a surgical solution for his back pain.

In May of 1961, President Kennedy arrived in Ottawa for a state visit, his first foreign trip following his inauguration. On May 16, the Kennedys were brought to Rideau Hall to join the Canadian Governor-General in a tree planting ceremony to mark his visit to Canada. With the cameras rolling, a youthful looking JFK grasped a shovel and began ceremoniously digging into a mound of dirt beside a pre-planted young oak tree. The moment was captured on film, but what the public didn’t see was that Kennedy had badly wrenched his back in the process. This led to further chronic and severe back pain, which Kennedy treated with a combination of medications that included hormones, animal organ cells, steroids, vitamins, enzymes, and amphetamines. He was reported to have suffered a number of side effects which included hyperactivity, hypertension, impaired judgment, nervousness, and mood swings. Kennedy's condition likely adversely affected his performance as president. He was taking a combination of drugs to treat severe back pain during the 1961 Vienna Summit with Soviet Premier Nikita Khrushchev, and his performance at the summit was not mentally sharp. Kennedy later acknowledged that he was not at his best during this summit.
Kennedy was regularly seen a number of doctors, one of whom was the controversial Dr. Max Jacobson, who was nicknamed "Dr. Feelgood" or "Miracle Max". Kennedy first visited Jacobson in September 1960, shortly before the 1960 presidential election debates. Jacobson was part of the Presidential entourage at the Vienna summit in 1961, where he administered injections to combat severe back pain. Kennedy was said to have ignored warnings from his other doctors about FDA reports on the contents of Jacobson’s injections and proclaimed. He is quoted as saying "I don’t care if it’s horse piss. It works." By May 1962, Jacobson had visited the White House to treat the President thirty-four times. The Bureau of Narcotics and Dangerous Drugs later seized Jacobson's supply of narcotics, and his medical license was revoked in 1975 by the New York State Board of Regents.
In December 1962, first lady Jacqueline Kennedy complained that her husband seemed depressed from taking antihistamines for food allergies. Kennedy took a prescribed antianxiety drug, Stelazine, for two days. Medical records show that at different times Kennedy took Codeine, Demerol and Methadone for pain; Ritalin, a stimulant; Meprobamate and Librium for anxiety; barbiturates for sleep; thyroid hormone; and injections of a blood derivative, gamma globulin, to combat infections. He sometimes received seven to eight injections of procaine in his back in the same sitting before news conferences and other events.

Towards the end of his life, Kennedy was reluctant to discontinue use of his back brace and crutches, even when recommended by physicians. Some have speculated that Kennedy's back pain may have been a contributing factor in his assassination in Dallas in 1963. When the first bullet struck him in the the neck, his back brace kept him erect, leaving him as an upright target for the next and fatal bullet which struck him in the head.