The study Vitamin D3 Is More Potent Than Vitamin D2 in Humans reported in the Journal of Clinical Endocrinology and Metabolism took a look at D2 versus D3 and came to the conclusion that
"Clearly, vitamin D3 is the preferable form of vitamin D."Not only is D2 is the less absorbable form of this vitamin, but the study concluded "taking 50,000IU of D2 should be equivalent to 15,000IU of D3 and perhaps closer to 5,000IU."
I was prescribed 50,000 units of vitamin D3 by a doctor per week for a vitamin D level of 30. Although vitamin D can be toxic in high dosages and needs to be monitored if you supplement at these levels, research suggests that blood levels of people with vitamin D insufficiency taking 50,000IU of D2 for 12 weeks raise the blood level by 25 ng while taking 50,000IU of D3 raised blood levels by 47 ng. so it's something to consider with the advise of your doctor if you have chronically low levels that don't change with usual supplementation.
If you do take this supplement regularly, you should have blood levels checked whenever you get a blood test to monitor whether you are absorbing it.
There are also drops that are 1,000IU and drops that are 5,000IU that you can take once a day. Posts below have more information on vitamin D. Also there is a fascinating ongoing Q&A about Vitamin D on The Vitamin D Solution.
Vitamin D--The Sunshine Vitamin
Vitamin D Drops for Infants, Kids and Adults
Vitamin D and the Flu