Without a detailed list of the medications your husband is taking there is no way to know for sure if SAMe would be detrimental for him. What I can tell you is that SAMe does have an excellent safety record and is not likely to interact with prescription medication in a negative way.
The average dose can range from 200mg per day up to 800mg mg per day. By nature, it has a very low toxicity and higher doses are usually tolerated well. The most common side effect noted is stomach upset. Some sensitive individuals can experience anxiety when SAMe is taken in high doses. Due to the fragile nature of your husband’s health, I would recommend starting on a low dose and slowly increasing over the course of 2-4 weeks to make sure he tolerates it well.
As far as how long to take it, there are no specific published guidelines. However, due to its excellent safety rating, I would not see a problem with taking it for an extended period of time in a healthy individual if it was helping.
SAMe can be converted by the body into a substance called Homocysteine. Homocysteine has been shown to be damaging to the arteries and contribute to heart disease. Taking adequate amounts of vitamin B6, B12 and folic acid will ensure that the body is able to recycle Homocysteine to its inactive counterparts rendering it completely safe. If you are concerned about this issue, just ask your doctor to give you a simple blood test to measure your Homocysteine levels. Many people in the population have elevated Homocysteine levels due to poor diet, excessive stress and exposure to toxins like alcohol and tobacco.