When it comes to coffee, there is a conflicting misconception that caffeine content is higher or lower depending on how coffee is roasted. Roast-levels however have no effect on caffeine content, and here is why.
Compared to a light roast, many people believe that a dark roasted coffee packs more punch, and as a result must contain more caffeine. This is true to some degree, but it isn't the roast that gives it an extra punch, it is the type of coffee bean being used. Most coffee, especially instant coffee bought on your supermarket shelves, contains a type of coffee bean called Robusta. This is a lower quality coffee bean that is easier and cheaper to produce for the mass market. It also contains more caffeine- in fact nearly twice as much as its Arabica counterpart!
There are also those that believe that a dark-roasted coffee must contain less caffeine, since the caffeine will be "burnt off". The only thing being burnt off, however, are all the interesting flavours that are all too often lost with this kind of roast-level.
The truth is that if you roast the same coffee bean light or dark, the caffeine content will be the same. The difference in caffeine content comes from the amount of coffee being used when brewing. Compared to a lighter roasted bean, dark-roasted coffee beans are lighter in mass because they have lost more of their moisture content as a result of roasting them to a higher temperature. For example, if you were to measure out 100g of each type of roast, you would end up with more dark-roasted coffee beans than the light-roasted variety. This difference can be small and usually only around 1-2%. To put this into perspective: if you measure out your coffee dose by weight, like you should do in order to get the best tasting coffee possible, there might be a difference of one or two coffee beans. As a result the darker roasted coffee will therefore have a slightly higher caffeine content- but we are talking about such a minute-percentile difference that in all respects they are pretty much equal.
So there you have it, the roast level of the coffee you drink isn't going to determine the amount of caffeine you are taking in. The biggest decision you have to make is: would you rather taste the deep complex, intense sweetness, fruity tanginess and high floral aroma that is representative of so many of our unique light or medium-roasted single-origin coffees, or the typically more-bitter, less-complex, roast-flavoured dark-roast coffee found in your average convenience store or gas station?