At Kirsten's suggestion, I'll  summarize what works for us to syringe metformin twice a day. Dr Kellon's suggestions are summarized in the metformin information in Files. I suggest starting with the right equipment before even trying to syringe. 
1. I bought a new inexpensive coffee grinder. It cost the same as good manual pill grinders and turns the hard 1000mg metformin tablets into powder much better. 
2. I bought the large size Wild Cherry Flavored Milk of Magnesia. 26 oz. lasts about a week. 
3. The missing link was reusable 60 cc catheter tip syringes with silicone O-rings. The plunger with a silicone O ring never sticks. These syringes are manufactured for feeding baby animals (Squirrel syringes) or for enteral feeding.  https://www.amazon.com/UrnConcern-Feeding-Syringe-Reusable-Silicone/dp/B081ZFZD5C/

I grind the metformin dose to a fine powder and pour the powder into 45 ml pre-measured milk of magnesia. I cover and shake a few times until the grittiness of the ground metformin is mostly gone, about 5-8 minutes. I draw half the mixture into each of two syringes. Dividing the dose allows me to comfortably position my thumb on the plunger and smoothly syringe one-handed. My horse is cooperative so long as I can syringe slowly in roughly 3 squirts per syringe. When we're done, I wipe her lips, especially the corners, with a very wet cloth. Then I rinse her mouth with a hose. I doubt this mixture of metformin and milk of magnesia causes sores, but undissolved metformin grit could, so I still rinse out her mouth. As a reward, I give my horse a tiny treat feed (about a cup of FORCO for now) to get the saliva flowing. 

Hope this helps anyone frustrated by disposable single-use syringes.
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Cass, Sonoma Co., CA 2012
ECIR Group Moderator
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