Signs and symptoms of mild cinchonism (which may occur from standard therapeutic doses of quinine) include flushed and sweaty skin, ringing of the ears (tinnitus), blurred vision, impaired hearing, confusion, reversible high-frequency hearing loss, headache, abdominal pain, rashes, drug-induced lichenoid reaction (lichenoid photosensitivity),[1] vertigo, dizziness, nausea, vomiting and diarrhea.

Large doses of quinine may lead to severe (but reversible) symptoms of cinchonism: skin rashes, deafness, somnolence, diminished visual acuity or blindness, anaphylactic shock, and disturbances in heart rhythm or conduction, and death from cardiotoxicity (damage to the heart).

Quinine may also trigger a rare form of hypersensitivity reaction in malaria patients, termed blackwater fever, that results in massive hemolysis, hemoglobinemia, hemoglobinuria, and kidney failure.

Attempted suicide by intake of a large dose of quinine has caused irreversible tunnel vision and very severe visual impairment.

[citation needed] Quinine, like chloroquine, inactivates enzymes in the lysosomes of cells and has an anti-inflammatory effect, hence its use in the treatment of rheumatoid arthritis.