DHT LEVELS AND FINASTERIDE

How do DHT levels change with one dose of finasteride?

Understanding how DHT levels change with finasteride is important in order to best counsel patients - especially those experiencing side effects. 

After taking 1 pills of finasteride, the drug itself is largely eliminated from the body in 1-2 days given that the half life of finasteride is about 4-6 hours (after five half lives a drug is significantly reduced in the body). However the same is not true of DHT levels. After a single dose of finasteride, DHT levels are reduced by 60 % (ie from 65 ng/dL to 25 ng/dL). However, the DHT levels don't rise back up quickly even though the drug is out of the body. Rather, DHT levels rise slowly increasingly just 15-20 % after the second day (ie from 25-30 ng/dL up to 30-35 ng/dL). Now back to the original question.

Of the two lines in the diagram, red or yellow, which best depicts how finasteride levels change with a single pill?

The answer is the yellow line! This concept is important since patients who are experiencing finasteride related side effects may still benefit from dosing every second or third day. It may not be quite as effective but as we can see from the graph, DHT levels are still being suppressed by this dosing schedule.
 

Reference

Vermeulen et al. Eur Urol 1991.