Cokes are rather unpalatable to me. Diet colas are also not very good due to their Aspartame content. Aspartame is the common ingredient in all fake sugar, Equal, Spoonful, etc.... which some people have had some pretty scary symptoms after drinking Aspartame such as: seizures, breathing difficulties, memory loss, nausea, headaches, and a host of other symptoms. Basically Aspartame can build up calcium in certain neurons on your brain, making if difficult for your brain to send messages to various parts of your body via these neurons. The neurons die due to over stimulation.
All that and it was just diet colas, now lets move on to regular colas. Sugar is a significant ingredient that will harm you. One can of cola contains 40.5 grams of sugar, that's around 10 teaspoons of sugar.
Colas also contain ethylene glycol, which is used in anti-freeze. Ethylene glycol is a slow poison, it won't kill you if you have it in small quantities but it will kill you in large quantities. That doesn't mean the small quantities are totally harmless however.
And the worst thing about colas are due to their extremely high acidity. The pH for colas is around 3.4, which scientifically is strong enough to dissolve teeth and bones. So, I'm not going to accept any coke if you offer me any.
All sources are from Google.com