"The median lethal dose (LD50 value) of vitamin A injected intramuscularly
in a water-miscible form in the young monkey is 0.6 mmol (168 mg) retinol/kg
body weight. Extrapolated to a 3-kg child and a 70-kg adult, the total LD50
dose would be 1.8 mmol (500 mg) and 41 mmol (11.8 g) respectively."
Also, it's teratogenic. Just like an innumerably many other chemicals we ingest daily.
Hence why Isotretinoin (http://en.wikipedia.org/wiki/Isotretinoin) is a restricted drug in many countries, although it is a vitamin A-like chemical that is used to clear up very severe acne. Women who are prescribed the drug in the UK have to provide proof that they are on a reliable form of contraception and are not pregnant when they collect each month's worth of pills. It causes severe birth defects.