I know that assisted suicide is sinful because it is taking matters into your own hands, but when is it a sin to refuse medical treatment, or is it sinful? I'm not talking about those who refuse it because they are going to keep right on praying that God will make them better, I'm talking about those who refuse it even though they know that if they got the treatment, they could have a chance of getting better, but if they refuse it, it won't make them better. You know, like refusing chemo, dialysis, stuff like that. If medical technology gets so advanced that we could live for hundreds of years, would it be sinful to say enough is enough, refuse something, and let ourselves go to be with the Lord because that is our desire?
First off, let's establish that there is nothing sinful about receiving medical treatment. See: Is It a Sin to Take Medicine? -- to Consult a Doctor?
But what about refusing treatment?
I know a few cases where a person dying of cancer was offered treatment, but that treatment had no hope of a cure. It might extend life a few more months, but it came at cost of pain and severe decrease in the quality of life because of the radiation and chemotherapy. I've known several people, when faced with those choices decide that they would rather enjoy the short time they have left on earth than to suffer just to stay here a short while longer. It isn't so much a refusal of treatment as a refusal to take a course that doesn't actually improve life or significantly extend life.
However, to refuse a treatment that offered a hope of a cure would be a different matter. For example, contracting pneumonia and then refusing to take the antibiotics that would likely eliminate the disease is basically putting God to the test. It would be similar to walking off of a cliff and trusting that God will prevent me from being injured. But that idea was soundly renounced. "Then the devil took Him up into the holy city, set Him on the pinnacle of the temple, and said to Him, "If You are the Son of God, throw Yourself down. For it is written: 'He shall give His angels charge over you,' and, 'In their hands they shall bear you up, Lest you dash your foot against a stone.'" Jesus said to him, "It is written again, 'You shall not tempt the LORD your God.'"" (Matthew 4:5-7). Man is not in the position to put God to the test.
When we die is in God's hands, but we still do what we can to either cure or minimize the suffering. As Jesus said, "Those who are well have no need of a physician, but those who are sick" (Mark 2:17). When you are sick, you should seek the help of a physician. Whether you will be cured is up to God, but we still do the small things that are available to us. Just as man is expected to work for a living (II Thessalonians 3:10), but to also remember to thank God for the blessing of food on his table (Matthew 6:11), so should a man in all his life do what he is able to do and then lean on His God to make up for all his inadequacies.
There is a time to live and and time to die, but there also is a a time to heal (Ecclesiastes 3:2-3). God scolded the Israelites for not seeking out healing for those sick among them. "The weak you have not strengthened, nor have you healed those who were sick, nor bound up the broken, nor brought back what was driven away, nor sought what was lost; but with force and cruelty you have ruled them" (Ezekiel 34:4).