taking a small amount wont stop sides. The difference in DHT between a fifth of a milligram and a whole 5 milligrams (25 times that dose) is only 7%. However variation in scalp DHT inhibition may have greater variation between doses so by taking a very small dose you are risking you potential success with the drug for no additional benefits other than the cost of taking less per day. If you are planning to add it to alchohol in order to apply it as a topical this will be of minimum effectiveness (pretty much almost nothing) as it has to be metabolized in the liver as the other guy said.
RU is different than finasteride I have never heard anyone say RU has to be metabolized in the liver and some very reputable people have had success with RU only (as a topical obviously).