Foods: Grapefruit juice Terfenadine

Some chemical exposures may be hepatic enzyme inducers/inhibitors, altering the effects of drugs like warfarin. Some foods such as mono-amines, will cause reactions in patients given MAOIs grapefruit juice may interact with terfenadine and some other drugs. New diseases may cause problems heart failure causing reduced liver blood flow can reduce the metabolism of drugs like warfarin. 

Many drugs will cause adverse effects if the dose is excessive but some cause adverse effects at the correct dose in certain individuals. This can be due to an increased sensitivity of the patient because of genetic or other factors (see pp. 29-31). There may also be side effects which are not apparent at low or therapeutic doses but which become more important at higher doses. Occasionally, unexpected and serious adverse effects occur in a few patients. Sometimes this is due to an interaction between two drugs or between a drug and a food constituent such as occurred with the drug terfenadine (used to treat hay fever) and grapefruit juice (see case note, pp. 33-4). 

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