The effect of drinking soft drinks is cumulative. Drinking one cup of soft drink doesn't do any long term damage but drinking it regularly does. Just like ink stain, you can wash it off but the stain remains. Soft drinks mostly contains water, lots of sugar, caffeine, phosphoric acid, colors and flavors. For example, the effect of phosphoric acid when it passes through the system is such that it causes a deficiency of magnesium, calcium, zinc etc. The damaging effect of sugar and caffeine also is not to be ignored. So the effect of consistently drinking soft drinks is long-term as these effects gradually progress and cause life threatening disease like diabetes.