According to Mayo Clinic, too much vitamin C in adults is any amount over 2 grams per day. The recommended dose of vitamin C for an adult is 60 to 95 milligrams a day.
Mayo Clinic explains that taking too much vitamin C is not harmful, but it may cause side effects, such as diarrhea, nausea, vomiting, heartburn, abdominal bloating and cramps, headache, insomnia, and kidney stones. It is possible to reach the recommended daily dose of vitamin C by eating a healthy diet. For instance, 1 cup of strawberries, red pepper or broccoli provides adequate vitamin C for one day.