Dexadrine over time/Pregnancy

Sorry..I had my drug name wrong.  I am on Dexadrine and have been for about 5 months.  While I am SOOOO HAPPY with the changes on the drug I have some concerns on the following....

The drug becomes obselete as you use it over time. Basically, you need more for it to work the same.  I have read that it is also highly addictive.  What have your doctors said on this topic?  We worked so hard to get the right one for me that I am panicing with the idea that I have to start over with another drug or the repercussions of taking a break on this one.

The other thing I am concerned about is getting pregnant while on this drug.  I want to get pregnant again and I have heard this drug causes birth defects in the developing lungs of a fetus....any info you can share let me know.