Addiction*: the compulsive need for and use of a habit-forming substance characterized by tolerance and by well-defined physiological symptoms of withdrawal.
or (broadly): persistent compulsive use of a substance known by the user to be harmful.
In order to appropriately answer the question, can trail running develop into an unhealthy addiction?, we must first look at the broader definition and ask ourselves, what constitutes an addiction?
Habit-forming: Breaking down the definition of addiction, it's easy to see how running, and specifically trail running, can become habit forming. When I run, my body releases endorphins. Endorphins give me a good feeling. I crave more of that feeling. With trail running, you have the added draw of being in nature, breathing fresh air, away from traffic and hustle & bustle.
Tolerance: I have built up a "tolerance" over my 13 years of running. I used to not be able to run a mile. Now I can run 50 miles. It took a while, but eventually 3 miles became "easy", then 6 miles, then 10 miles. The more you run, the more your body adapts to running and the easier it becomes. Just like any practiced skill.
Withdrawal: Some people may disagree with me, but I believe there are physiological symptoms of withdrawal from running. When I don't run for a day, I am cranky. People probably don't really want to be around me, I'm not pleasant or fun. More than 3 days and I'm sluggish and feel bloated and foggy. I don't sleep well if I haven't run and my appetite and energy levels are poor.
Compulsive: I think the key to uncovering our answer is to determine if trail running is compulsive. Do I feel compelled to run trails? Is it an obsession? When considering trail running, I feel it is more of a calling. When I haven't hit the trails in a few weeks due to my work schedule or family obligations, I feel the trail calling. I have a strong desire to be out in the wilderness, alone with nature. But, yes, I suppose you could say I'm compelled to run trails. It's a force stronger than my will to resist.
Harmful: And then we have to address the "harmful" quality. Is trail running harmful? Can harm come to me by trail running? I fall on trails occasionally. Knock on wood I have not been seriously injured, just a few bruises and flesh wounds. (Have I shown you my scar? I'm rather fond of it actually...) I could be bitten by a rattlesnake, or if I lived in a different part of the country, mauled by a mountain lion or bear. That would be harmful. I could sustain overuse injuries to my bones, joints, etc if I were not so diligent about injury prevention. And then there's the not-so-physical connotation of the word harmful. I could harm my relationships with family or friends if I spent every waking moment on the trails, ignoring my relationships. I could be fired from my job if I called out of work every day to spend time trail running. I could harm my social life by avoiding social situations after 7 pm and analyzing every morsel of food I put into my mouth, I mean, who wants to go to happy hour with "that girl"? (Disclosure: my bed time is actually 8 pm, not 7.)
Unhealthy: And the final key to answering our mystery question is to determine if trail running is unhealthy. I try to imagine a time in the not-so-distant future when I will be training for my first 100 mile trail run. I will spend hours every week on the trail, and running. Hours preparing mentally. Hours preparing my gear. Researching the course. Planning. And then the long weekend spent selfishly away from life during the execution of said 100 mile run. For me, I do not believe trail running is unhealthy. For someone else, maybe. Someone trying to force their body to do something they should not do for medical reasons, technically, is unhealthy.
So I guess, the bottom line is, YES, trail running can develop into an unhealthy addiction. (Can you feel my arm being twisted on this one?) But that doesn't mean it is always unhealthy, or always an addiction, or always an unhealthy addiction. Sometimes it's exactly what you need to help keep you sane. It might be just what the doctor ordered to manage stress, or weight, or cholesterol, or whatever. So until I've broken both femurs and the mountain lion drags me off the trail... I'm going to keep trail running.
*As defined by Merriam-Webster Online Dictionary