Hello Chloroquine used as an antimalarial is a relatively safe drug. Commonly it causes nausea & vomiting (as in your father case). The drug , however , may produce serious toxicity when used in LARGE DOSES for prolonged periods as in rheumatoid arthritis. Vomiting and nausea with abdominal discomfort is common with chloroquine. When such patient visit I advise them to take pantoprazole-d to avoid nausea and vomiting . Don t take this tablet on empty stomach . Usually good result is obtained when taken after meal with milk. So ,if your father feel untoward effects ,consult your doctor , otherwise use this medicine as advised.