I don't think it's inappropriate to question it once the subject is out there. Though, once you've questioned it, the onus is on you to provide the rationale/evidence to justify the larger doseage vs. any risk factors it may present. Most of the risks are not considered negatives to trans-women (Though most men would consider them negatives), so you may have a good chance of the doctor complying with your reuqest as long as you're upfront about your future plans.
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Personally, I've been on the higher 'enlarged prostate' dosage level for almost 3 years now. I've never used the lower doseage. It hasn't given me any problems, but your doctor may think differently for you depending on your history/chart. I'm divided about whether I think it's helpful. I haven't lost any more hair...but whether that would have happened without Fin is impossible to say. Another positive about the larger doseage is that unlike the MPB doseage, the larger doseage is usually covered by insurance. (It's the only HRT med I take that isn't completely out-of-pocket)
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Best of Luck,
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Kay