I keep seeing ads for this Avectar something or other weight-loss product and what it claims to do is amazing. I'm curious as to how weight-loss supplements affect birth control users. Birth control is my main form of contraception.

no they do not
only antibiotics

you should very carefull and ask your doctor and read the labels...
for fast weight loss... you have to supplement on top eating well and excercise...im not sure what effect this has on birth pills
make sure you take a good multivitamin
and maybe consider a thermogenic fat burner pill
also green tea and lots of water is good.
but if its quick weight loss, then hoodia pills are the way to go, as featured on oprah and 60 minutes....
i have tired many different types of hoodia including gordonii you can buy these online but some online stores are suspect so be extra careful. i have personally had better results with hoodia+green teas so i highly reccommend it.
i think you should try out green-tea hoodia also
you can save alot of money by using the free trial offers offered by most vendors like these below (i only have to pay shipping)...
http://bytead.com/free-7days-hoodia/
http://bytead.com/free-15days-hoodia/
overall this will be a temporary way to get over a weightloss plateau... but it will work...
take care :)