You'll need an analytical balance for that....probably one with an enclosed cabinet. I've always found that air movement will affect readings below 25mg or so.
Another thought is to figure out some way to measure a larger amount of the drug - for example, measuring 100mg of it, then dissolve it in 10ml of water...using 1ml of that solution would give you 10mg of chemical, much more accurately than trying to weigh out such a tiny amount.
Jay