This question makes a few assumptions.  One, that we have the best medicine ever. Two, that people described in the Word of God lived for longer periods of time without the benefits of medicine.

The Word doesn’t say that people before the flood who lived past 900 years did not use medicine. However, what we do see is after the flood the years began to rapidly digress to what we see as normal today.  About 65 years ago the life expectancy was about 48 and increased to about 67 in 2010.  Even though we have examples in the Word of people living past 120, according to a Psalm by Moses, the expectancy was 70 years of life and 80 years would be an exception [Psalm 90:10].

The Word doesn’t give specifics about why or how people lived longer which leaves a lot of room for speculation.  One thing that should not be assumed is that there was no beneficial medicine in those days.

There’s a great article that points out the medical knowledge that can be found in the Word.  It discusses the eighth day being the best day for circumcision due to blood clotting in newborns.  Also, documents the medical benefits of biblical water purification, diet restrictions, leprosy management, and more.  Even noting some of the practices were not discovered (or rediscovered) in modern times until fairly recently.  So, it may not be wise to assume we have the best medicine ever:

Here are a few facts:

• Our medicine is great in a lot of respects but also has caused an “epidemic” according to the CDC
• “The unfortunate circumstance around so many prescription drug overdoses is that most victims never even knew they OD’d.” Read more.
• “Every day, 44 people in the U.S. die from overdose of prescription painkillers, and many more become addicted.” CDC

This tells us that a leading cause of death comes from medicine that is LEGALLY prescribed to us by our healthcare providers.