I've read more than one post stating that their hair had thinned or had gotten lank and lost it's shine while on opiates in general. This is just my guess, but I imagine if someone is taking a high dose of opiates, your overall health is affected, and if you're not healthy on the inside, it will show on the outside.
Others have indicated that their nails would break more easily, and their skin was always dry or itchy. It's possible that opiates affect absorption of proteins that would keep one's hair and nails healthy and strong.
The good news was that, once they stopped the opiates, the situation reversed itself. Healthy hair and nails came back, etc.
Well I don't know of any specific studies done but I can tell you my hair began to thin after I'd been on tramadol for a year or so, and since I quit it's come back to the way it used to be. I don't know if there's a correlation to that but it sure felt like it.