I did my DaTSCAN last year and have no special negative memories of the needles part (and I'm no needle fan either).
In what regards your question on meds interactions, you may want to look at the following link, which provides an interesting overview of DatScan: http://www.datscan.cz/files/firemni/003009_low_res.pdf
As you'll see, page 17 says (after INTERACTIONS): "Drugs shown during clinical trials not to interfere with DaTSCAN imaging include amantadine, trihexyphenidyl, budipine, levodopa, metoprolol, primidone, propranolol and selegiline. Dopamine agonists and antagonists acting on the postsynaptic dopamine receptors are not expected to interfere with DaTSCAN imaging and can therefore be
continued if desired."... which is consistent with turnip's explanation.
I guess you should, of course, tell the doctors that do the scan about your meds... and if you prefer to play it safe anyway, maybe still worth calling them in advance and just ask their views.