A. Most medications do pass into breastmilk, but that doesn't always mean that you can't take them. Unfortunately, the Physicians Desk Reference, which most doctors use to determine if a drug is safe while you are breastfeeding, rarely has good information about drugs and breastfeeding.
For Imitrex, the PDR states that 'Sumatriptan is excreted in human breast milk. Therefore, caution should be exercised when considering the administration of IMITREX Tablets to a nursing woman.'
Many doctors would read that and feel uncomfortable recommending that you continue to breastfeed while taking the medicine, partly because they aren't sure it is safe and partly from a fear of being sued if you take it, continue to breastfeed and something happens.
A little more searching though, and you find many references that indicate that it is likely safe to breastfeed and take Imitrex. According to the American Academy of Pediatrics, 'Most drugs likely to be prescribed to the nursing mother should have no effect on milk supply or on infant well-being.'
One good source of information concerning drugs and breastfeeding is the AAP policy statement on The Transfer of Drugs and Other Chemicals Into Human Milk. This guide lists Imitrex (sumatriptan) as a Maternal Medication Usually Compatible With Breastfeeding (table 6).
This babycenter.com guide to Drug Safety During Breastfeeding also lists Imitrex as a medicine that is Probably Safe in Usual Doses.
While breastfeeding, it can be helpful to have a doctor that is supportive of breastfeeding so that if it does become necessary to take a medication, then a drug is chosen that is safe for you and your baby. If your doctor says that the drug is not safe to take while breastfeeding, then ask for an alternative. Printing out the above lists from the AAP and babycenter.com and taking them to your doctor visits might also help your doctor choose a safe medicine.
If you are still concerned, this advice, also from the AAP might also help decrease any possible risks to your baby. You can usually decrease a baby's exposure to a drug 'by having the mother take the medication just after she has breastfed the infant or just before the infant is due to have a lengthy sleep period.'