Caffeine: Coffee, tea, and soft drinks with caffeine should be limited. Caffeine does pass through the milk and makes some babies restless and fussy.
Alcohol: Alcohol is a drug. It passes through the milk to your baby. If you do choose to drink alcohol, do not do it often, and do it soon after you nurse, not right before.
Nicotine: Smoking will cause you to make less milk and the milk will have less vitamin C. You are adviced to quit smoking. If not, you should still breastfeed. It is better to smoke after nursing, not before. Do not let anyone smoke near your baby.
Illegal Drugs: All illegal drugs (like marijuana, heroin and cocaine) will hurt you and your baby.
Medication: If you are taking a prescription, talk to your doctor. Many medications are safe during breastfeeding, while others are not. For example, there are some anti-depressants and birth control pills that are safer than others for nursing moms.
Source: “Why Should I Nurse My Baby? & Other Questions About Breastfeeding” Pamela K. Wiggins, IBCLC & Joanna Wiggins Garofalo.