excellent places to find accurate resources about medication and breastfeeding:

Dr. Hale http://neonatal.ttuhsc.edu/lact/ check out the medication forums. and also he has a book called "Medications and Mothers Milk"
you will find most basic pharmacology guides, sites and pharmacists as well as doctors do not have the accurate information about the effects on nursing babies, and the mothers milk. Dr. Hale studies this exclusively.
Dr. Newman and Linda Smith have some good information on their site
http://www.bflrc.com/
http://www.bflrc.com/newman/articles.htm
http://www.bflrc.com/ljs/ls-arts.htm

Mothering Magazine ran an article by Dr.Hale a while back, here is the link
http://www.mothering.com/12-0-0/html/12-4-0/bfeeding-medications.shtml

Table 1. Partial list of medications of concern or those contraindicated (that means, these meds you watch to see if there are any side effects, and alot depends on your baby's age and size. as well consider if they are exclusively breastfed or are taking solids as well.)
Drug: Effect on lactation/infant
ACE inhibitors: High risk of hypotension in young neonates but no problem for older infants
Acebutolol: Low blood pressure, low glucose levels, and breathing difficulties (apnea)
Amphetamines: Loss of appetite, agitation; risk does not justify use
Anticancer agents: Possible immunosuppression/toxicity in neonate
Barbiturates: Monitor for infant sedation
Benzodiazepines (Valium drugs): Chronic use may lead to infant sedation and/or dependence
Bromocriptine (Parlodel): Inhibits lactation; suppresses prolactin
Cabergoline (Dostinex): Inhibits lactation and prolactin
Cocaine: Infant intoxication
Ergotamine: Inhibits lactation and prolactin
Estrogens: Suppresses lactation; use with caution
Fluoroquinolones: Some may produce bloody diarrhea
Lithium: Monitor maternal/infant plasma levels and thyroid function; use with great caution
Lovastatin and others: Lowers cholesterol; risk does not justify use
Methotrexate: Possible immunosuppression; loss of white blood cells; accumulation in gastrointestinal tract
NSAIDS: Avoid prolonged use of long half-life NSAIDS; GI distress, diarrhea
Antipsychotics: May induce sedation, increase risk of apnea
Radioactive Iodine-131: Accumulation in milk/breasts; thyroid toxicity/carcinoma

Table 2. Radioactive and other medications for which temporary pumping and discarding of milk is recommended
Medication: Recommended Period of Interrupted Breastfeeding
Radioactive Iodine-131: Complete cessation
Radioactive Iodine-123: 24 hrs for 10 mCi(millicurie); 12 hours for 4 mCi
Radioactive Iodine-125: Complete cessation
Tc-99m Pertechnetate: 24 hrs for 30 mCi; 12 hrs for 12 mCi
Tc-99m Sulfur Colloid: 6 hrs for 12 mCi
Tc-99m WBC: 24 hrs for 5 mCi; 12 hrs for 2 mCi
Gallium-67: 1 month for 4 mCi; 2 weeks for 1.3 mCi; 1 week for 0.2 mCi
Indium-111: 1 week for 0.5 mCi
Thallium-201: 24-48 hrs following 111 MBq (megabecquerel)
Cisplatinin: 3-7 days postinfusion
Cocaine: 24 hours
Metronidazole: 12-24 hours following 2-gram dose only
Doxorubicin: Complete cessation
Copper-64: 50 hours

Table 3. Some of many medications considered safe for use by breastfeeding mothers
Penicillin antibiotics
Cephalosporin antibiotics
Flagyl (Metronidazole)
Reglan (metoclopramide)
Zoloft (sertraline)
Paxil (paroxetine)
Motrin, Advil (Ibuprofen)
Tylenol (acetaminophen)
Inderal (propranolol)
Zithromax (azithromycin)
Erythromycin antibiotics
Codeine
Morphine at moderate doses
Pepcid(famotidine)
Prilosec (omeprazole)
Heparin
Insulin
Diflucan (fluconazole)
All vaccines

Ideal drug characteristics for breastfeeding mothers:
Drugs with shorter half-lives
Drugs with poor oral absorption
Drugs low in toxicity
Drugs that are non-sedating

Suggestions for breastfeeding mothers
Use medications only when necessary.
Be flexible; choose medications that are preferred for breastfeeding mothers.
Medications cleared by the FDA for infants are generally safe for breastfeeding mothers, too.
Avoid maternal peak blood levels when breastfeeding.
Use drugs that are poorly absorbed or inactive orally.
Use drugs with shorter half-lives.

from:
http://www.mothering.com/12-0-0/html/12-4-0/bfeeding-medications.shtml