When you are breastfeeding, you are not supposed to take any decongestants to relieve cold or allergy symptoms. I’ve been breastfeeding for over a year now, and I’ve had to deal with both. As a matter of fact, I have a cold now. Joy! 
A few months ago, I discovered applying 100% peppermint oil really cleared me right up. I don’t know if this will work for everyone, but it works for me.
At first, I was heating the oil in an oil burner. It made the room smell like a candy cane and relieved some of my congestion. I needed something a little stronger, so I applied it to side of and below each nostril. The skin on my nose felt irritatingly tingly, but didn’t burn. Then, my nose started to run. Whoo! What a relief. I was able to sleep after that.
If you try this, do not get it in your eyes or mouth or inside your nostrils! It’s not supposed to be ingested. You only need to dab a miniscule amount–less than a drop–to get results. I recommend testing this out to make sure you’re not allergic or become overly irritated.
Although the mint oil works great alone, applying the peppermint oil in conjunction with the use of a humidifier gives even better results. I find drinking peppermint tea is also good. I prefer Twinings pure peppermint tea, because it is very minty.
Hope this helps.
