Your answer is very standard. There are a lot of diseases where no research is being done in the USA like progressive MS. RRMS is the preferred drug target and so all drugs marketed are for that favor of MS. These RRMS drugs have no effect on the progressive form of the disease. Taking Viagra is worth a try if taken with caution. Many people already self medicate by taking vitamins and/or supplements. You also need to be careful taking vitamins. As long as you are careful, then trying Viagra which has a very good safely record is worth a try. It will probably take about 15 years before the FDA could investigate Viagra relationship to MS. I hope that people with MS do not listen to a your very standard answer and try a very hopeful new approach for treating a extremely bad disease.
Though Viagra has no specific side effects pertaining to MS, it is only advised to use the medication if advised by a medical doctor and only under the prescribed doses. Unnecessary self-medication is more harmful than useful in most cases and is best avoided.
Hope the information is helpful.
Take care!