How unsafe is a one-day megadose of vitamin a, taken to help knock out a nascent cold? (24, 000 i.u., then 24, 000 8 hrs. later).

Not likely to help: A one-time dose like this is unlikely to kill, but also unlikely to help. It is an entirely unnatural amount of vitamin a to ingest, and there is some evidence that taking vitamin a supplements could be harmful to the heart and the bones, and could increase risk of cancer. This would be much more likely for daily use than from one-time use, but not worth the risk in my opinion.

Not helpful: Vitamin a has not been shown to help with the common cold (uri). Excessive vitamin a can cause pseudotumor cerebri and consequent blindness.

No harm: These dosages are not going to cause any harm. However, there is no data to support the use of vitamina as a supressant of a cold or upper respiratory infection. You are supporting the supplement industry but not yourself.