Red eye coffee can be made with milk, and this will not affect the caffeine content. The milk will help to mellow out the coffee and make it less bitter.  You can also add sugar or other sweeteners to taste. If you want a stronger coffee, you can add an additional shot of espresso. However, this will also increase the caffeine content.  If you don't want to use regular milk, you can opt for soy, almond, or cashew milk instead. Just make sure to adjust the amount of sweetener you add accordingly.