Drifter's studiness isn't a huge issue - Heather says he's about as well-behaved as a well-trained stud. But he is easily distractable, particularly by the presence of other horses, and it probably makes him a bit more of a tester - he's always trying to see if he can test the limits, both of people and of horses he's turned out with, although of course some of this is learned behavior. Our vet/chiro had suggested that we might try him on a low dose of depo-provera - she's had good luck with that in treating studdy geldings. An alternative (very inexpensive), suggested by my trainer Heather - she's had good luck with it - and confirmed by our vet/chiro, is Chaste Tree berry. This affects pituitary function, and can have a beneficial effect for horses like Drifter, allowing them to be a little less mentally scattered and reducing some of the studliness.
Chaste Tree berry is also often used in horses with Cushing's, often with good results (based on what I've read and what our vet/chiro says - I have no personal experience with it). Interestingly enough, Drifter has been very slow to shed out and his coat is still very thick and a bit curly, and he drinks a very large amount - often a sign of an issue with pituitary function. Pie's been shedding for a month and Dawn's well on the way, but not Drifter, despite our very warm weather. Drifter is young enough (he's 10) that real Cushing's is unlikely, but our vet/chiro says that the EPM organism does have pituitary suppression effects in certain cases, so it could be that this is a lingering effect of the EPM.
In any event, the chaste tree berry is ordered, and Drifter will be started on two teaspoons a day. We'll see how he responds to that - it's worth a try.