Morphine has a high potential for addiction; tolerance and psychological dependence develop rapidly, although physiological dependence may take several months to develop. Tolerance to respiratory depression and euphoria develops more rapidly than tolerance to analgesia, and many chronic pain patients are therefore maintained on a stable dose for years. However, its effects can also reverse fairly rapidly, worsening pain through hyperalgesia.

Immediate release morphine is beneficial in reducing the symptom of acute shortness of breath due to both cancer and non-cancer causes. In the setting of breathlessness at rest or on minimal exertion from conditions such as advanced cancer or end-stage cardio-respiratory diseases, regular, low-dose sustained release morphine significantly reduces breathlessness safely, with its benefits maintained over time.

The structure-activity relationship of morphine has been extensively studied. As a result of the extensive study and use of this molecule, more than 250 morphine derivatives (also counting codeine and related drugs) have been developed since the last quarter of the 19th century. These drugs range from 25% the analgesic strength of codeine (or slightly more than 2% of the strength of morphine) to several thousand times the strength of morphine, to powerful opioid antagonists, including naloxone (Narcan^®), naltrexone (Trexan^®), diprenorphine (M5050, the reversing agent for the Immobilon^® dart) and nalorphine (Nalline^®). Some opioid agonist-antagonists, partial agonists, and inverse agonists are also derived from morphine. The receptor-activation profile of the semi-synthetic morphine derivatives varies widely and some, like apomorphine are devoid of narcotic effects.

Effective plasma levels vary depending on the medical indication. For congestive heart failure, levels between 0.5 and 1.0 ng/ml are recommended. This recommendation is based on post hoc analysis of prospective trials, suggesting higher levels may be associated with increased mortality rates. For heart rate control (atrial fibrillation), plasma levels are less defined and are generally titrated to a goal heart rate. Typically, digoxin levels are considered therapeutic for heart rate control between 1.0 and 2.0 ng/ml. In suspected toxicity or ineffectiveness, digoxin levels should be monitored. Plasma potassium levels also need to be closely controlled (see side effects below).

Researchers at Yale University looked at data from an earlier study to see if digoxin affected men and women differently. That study determined digoxin, which has been used for centuries and makes the heart contract more forcefully, did not reduce deaths overall, but did result in less hospitalization. Researcher Dr. Harlan Krumholz said they were surprised to find women in the study who took digoxin died «more frequently» (33%) than women who took a placebo pill (29%). They calculated digoxin increased the risk of death in women by 23%. There was no difference in the death rate for men in the study.