I have psoriasis on my penis and was told not to use betamethasone, which clears it up quickly. why not, since it works?

Risk of atrophy: Steroids are classified by there potency. For example Betamethasone dipropionate is high and Betamethasone valerate is mid. Potency is also altered by whether the steroid is an ointment or a cream. Short periods of treatment are okay but extended use of potent steroids increases risk of irreversible skin atrophy (thinning). Therefore very important to follow advice of a dermatologist.