Vitamin C and magnesium are great for moving the bowels. They are quite safe in large dosages. The side effect of overdosing is diarrhea.
You sound like you are impacted. Stick with the program. You will get there.
If you are up to 18 and still only having one bowel movement a day, this should tell you that you have a need for the program.
Many people are deficient in magnesium and it would only benefit you to take it. It is also good for the heart. Take the vitamin C also.