I've suffered with seasonal allergies all my life. It was not uncommon for my nose to run, especially in the spring & fall. Since starting Mestinon, for about 30 minutes after taking my first dose of the day, I usually have a mild runny nose. It only happens with my first dose. Also as Desert Flower has mentioned, plenty of water helps many of the side effects.

As others have mentioned, a Mestinon allergy or overdose is always a possiblity.
Be sure to mention this to your neuro...

Rachel