Phentermine Effects on Breast Feeding Information

I have a five-month-old little girl but she usually only nurses for comfort. I began taking phentermine today to lose weight and took two halves of a 37.5 mg.(my previous diet dose). I didn't nurse her of course, but now realize I've made a mistake and neither one of us is ready for this special relationship to end. I don't want her to get any of this drug through my milk. How soon can I safely resume nursing her again?


Debbi Donovan

Debbi Donovan is a Board Certified Lactation Consultant, as well as a retired La Leche League Leader. For more than a decade, Debbi... Read more

Phentermine is an appetite suppressant for the short-term treatment of obesity. Because of its small molecular weight (149), this drug probably transfers readily and in significant quantity into human milk. It could cause stimulation, anorexia, tremors and other central nervous system effects in the nursing baby. This medication should not be used by nursing mothers (Hale 1997).

This medication has often been prescribed along with fenfluramine -- this drug combo is popularly know as fen-phen. Researchers reported in the New England Journal of Medicine, that fen-phen (fenfluramine and phentermine) may be responsible for heart damage in 24 women with no previous history of cardiac disease. Due to the potential for very serious side effects, it is cautioned that these drugs should only be used by people whose obesity poses a serious health risk or is combined with heart disease, high blood pressure, diabetes, or other health problems. (The New England Journal of Medicine.1997;337:602-606, 629-630, 635)

Phentermine has an adult half-life of between 7 and 20 hours, and peaks in the bloodstream (and most likely in your milk) 8 hours following administration. Assuming a seven hour half-life, over 90% of the medication would theoretically be cleared from your system in 28 hours. Assuming a 20 hour half-life, it would take approximately 80 hours for most to clear. I would recommend discussing your concerns (immediately) with your Pharmacist or Health Care Provider, so you can come to a well-informed decision.

If you do decide to wait a bit to nurse your baby, I would highly recommend expressing your milk as often as she would normally nurse. If you don't express, your supply will begin to diminish and the sodium content of your milk will start to increase. (Your little one might refuse to nurse again if she takes a sip, and your milk tastes salty.) Do not use the milk you express at this time.

Even if you have taken a two or three day break from nursing, especially if you have expressed your milk, there is still a chance that with tender loving care your little one will return to your breast.

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