There are people who claim the possibility of kidney stones from too much vitamin c. Vitamin c also increases iron absorbtion, so those who eat plenty of iron and take large amounts of vitamin c might absorb too much iron. This is more of a problem for men than women though. I would advise against taking 50 grams a day of vitamin c. Imo up to 5 grams on a long term basis, or 12 or so grams for a few days during acute illness makes sense. This page is interesting.

http://en.wikipedia.org/wiki/Vitamin_C

Even if 10 grams-20 grams of vitamin c a day doesn't hurt, I can't see how it would help most people much more than 5 grams a day.

As for herbs that have a laxative action, I suggest that people avoid senna, buckthorn, etc. especially long term, as these are not safe to use long term. They also tend to cause rebound constipation. I like using herbs that have a more mild laxative action like licorice root,garlic, ginger, alfalfa, and dandelion.These herbs are also great for detoxification. Eating more vegetables(and fruits for those who don't have candida issues) and drinking more water can help greatly. So can exercise. I also read that parasites cause constipation.
Taking probiotics will also help with constipation. The link below is interesting. I had forgotten about olive oil as a laxative. Olive oil is a good choice for long term constipation prevention. Some like using it raw, while others like to cook with it.

http://www.health911.com/remedies/rem_consti.htm