Chloroquine and hydroxychloroquine are similar in several ways. Both are derived from the quinoline molecule, both are antimalarial, and both are used to treat conditions such as rheumatoid arthritis and lupus erythematosus. The most significant difference between the two compounds is widely accepted to be their toxicity levels. Chloroquine is believed to be significantly more toxic than hydroxychloroquine and is more likely to lead to serious side effects. In particular, ocular toxicity is believed to increase the risk of severe and irreversible eye damage, particularly with long-term use. This is why hydroxychloroquine is preferred for patients who are likely to take the drug for life, such as those living with lupus erythematosus. Additionally, the greater toxicity of chloroquine increases the risk of overdosing, which can be fatal.