Finasteride was developed in the 1970s and was FDA approved in 1992 to treat BPH and later male pattern hair loss in 1998. Dutasteride is a much newer drug and was approved to treat BPH in 2001 and is yet to be FDA approved for hair loss. Both drugs belong to a class of medications called 5-alpha-reductase inhibitors, or 5-ARIs. 5-ARIs inhibit the conversion of testosterone to the androgen dihydrotestosterone (DHT), which is the primary cause of male pattern baldness.
The main difference between finasteride and dutasteride is that finasteride is selective for 5AR type II while dutasteride inhibits both type I and type II. Dutasteride also has a longer half-life of 5 weeks compared to the half-life of 5-6 hours for finasteride. Dutasteride was found to be 3 times more potent than finasteride at inhibiting 5AR type II and 100 times better at inhibiting 5AR type I. One reason may be because dutasteride blocks 90% of DHT compared to 70% by finasteride, indicating that oral dutasteride may be more efficacious than topical and oral finasteride. However, topical finasteride may be beneficial if you have taken oral finasteride and experienced adverse side effects. Topical finasteride is applied directly to the scalp, which may lower systemic side effects like sexual dysfunction.
Dutasteride is only available in one dosage form: liquid-filled capsules to treat BPH. There has been interest in the idea of topical dutasteride, but there are currently no studies published regarding its effectiveness.
