Yes, it could.

Alli, a form of the drug orlistat, works by blocking fat absorption from the intestine. (This leads to some nasty diarrhea if people who take it don't stick to a low-fat diet!) It's a really bad idea for breast-feeding moms to take it, for two reasons:

• Some vitamins are fat-soluble, meaning your body needs to ingest fat in order to get the vitamins. These include vitamins A, D, E, and K, along with beta-carotene. People on orlistat can end up not getting enough of these important nutrients -- and since breastfed babies rely on breast milk to get these vitamins, they could end up not getting enough as well.
• It's not known for sure if orlistat gets into the breast milk of moms who take it, but most medications do, at least in small amounts. If the baby ends up getting some Alli this way, it could interfere with the baby's ability to digest fat. This could be very dangerous. Fat is crucial for the baby's growth and brain development.

For these reasons, the product packaging is pretty clear that orlistat shouldn't be taken while breastfeeding.

Remember, breastfeeding burns extra calories and can be a great way to get started on shedding pregnancy pounds. Instead of using medications to lose weight, stick to a healthy diet of fruits, vegetables, whole grains, and low-fat meat and dairy. Get some exercise every day -- it can be hard to go to the gym when you are a new mom, but a daily walk is always possible. Bring the baby along -- and burn extra calories -- by putting him or her in a baby carrier or a stroller.