If you have a DPD enzyme deficiency, one dose of 5-FU chemotherapy can kill you
A good article written by Chris at "Chris Beat Cancer" discusses the toxicity induce by 5-FU that can be lethal for those missing a liver enzyme called Dihydropyrimidine Dehydrogenase (DPD).
It seems that DPD deficiency affects 5% of the population which could be at risk of death from only 1 dose of 5-FU. An additional 3% to 5% of the population has a partial DPD deficiency.
There is a blood test that costs around $100 to check for the DPD enzyme deficiency.