Lovesickness is the mental state brought on by the personal experience of unrequited love, or unrequited limerence (also known as infatuated love or being smitten). Research on the biology of romantic love indicates that the early stage of intense romantic love (also called passionate love) resembles addiction, but academics do not currently agree on how love addiction is defined. Lovesickness is characterized by addictive cravings, depression and intrusive thoughts about a love object.

The term "lovesickness" is rarely used in modern medicine, though new research is emerging on the impact of heartbreak on the body and mind.

In the medical texts of ancient Greece and Rome, lovesickness was characterized as a "depressive" disease, "typified by sadness, insomnia, despondency, dejection, physical debility, and blinking." In Hippocratic texts, "love melancholy" is expected as a result of passionate love. Lovesickness could be cured through the acquisition of the person of interest, such as in the case of Prince Antiochus.

In ancient literature, however, lovesickness manifested itself in "violent and manic" behavior. In ancient Greece, Euripides' play Medea portrays Medea's descent into "violence and mania" as a result of her lovesickness for Jason; meanwhile, in ancient Rome, Virgil's Dido has a manic reaction to the betrayal of her lover, Aeneas, and commits suicide. Dido's case is especially interesting, as the cause of her lovesickness is attributed to the meddling of the gods Juno and Venus.

In the Middle Ages, unrequited love was considered "a trauma which, for the medieval melancholic, was difficult to relieve." Treatments included light therapy, rest, exposure to nature, and a diet of lamb, lettuce, fish, eggs, and ripe fruit.

In both antiquity and the Middle Ages, lovesickness was often explained by an imbalance in the humors. An excess of black bile, the humor correlated with melancholy, was usually considered the cause.

In 1915, Sigmund Freud asked rhetorically, "Isn't what we mean by 'falling in love' a kind of sickness and craziness, an illusion, a blindness to what the loved person is really like?"

Scientific study on the topic of lovesickness has found that those in love experience a kind of high similar to that caused by illicit drugs such as cocaine. In the brain, certain neurotransmitters — phenethylamine, dopamine, norepinephrine and oxytocin — elicit the feeling of high from "love" or "falling in love" using twelve different regions of the brain. These neurotransmitters mimic the feeling of amphetamines.