Running on a treadmill is different to running outside so run on it as little as possible. If you do most of your runs outside and like to have personal best times for distances, then don't switch to, and run too much on a treadmill. If your personal best was measured from outside running then it makes sense to train your outside running to improve it.
It can be difficult if you've bought a treadmill or have a gym membership to avoid using one. Maybe you want to get the most use of it as you can and there are times when using a treadmill is a good idea. But it isn't in normal training. There's certainly no evidence that it does slow you down apart from certain runners saying it does for them.
When to train on a treadmill
If the weather outside is that bad or you don't feel safe at night and other reasons then a treadmill is ideal to keep your fitness levels up. You get that run you wouldn't have been able to do if you didn't have access to a treadmill. To make it similar to running outside it is recommended you set the incline at 1% for a similar cost in energy used.
There are circumstances when training on a treadmill is ideal and that is when you want to do some speed work. This is when the treadmill helps you run faster. By doing speed work you get your body used to exerting a lot more energy as you run and you'll be able to do your normal runs quicker. We have 3 pages of speed workouts specially made for the treadmill:
We don't really know if training on a treadmill will slow you down. For some it does and others it doesn't. So don't take the risk is our advice. Do the least amount of normal runs on a treadmill as possible, to the point where the question of whether it slows you down doesn't enter your thoughts. As you don't use it enough to find out.
Treadmills have their uses as we've covered. They're great for beginners and sometimes with minor injuries experienced runners can find them useful as well. They come into their own when it comes to speed work and we recommend you give it a try if you haven't. The treadmill is by no means redundant but if you like to run outside for your training then stick to what you like doing and use a treadmill only when you have to.