I use the syringe and feel the burn for about 10 seconds when medicine is being injected but then the pain stops and all is well! I have not had any flu like symptoms or any adverse side effects other than a few days after the injections I get very itchy all over my body, this lasts about a day and then disappears. I take Benadryl and I'm just fine. Don't worry, it's not a huge deal. Take care and be strong!