it is perfectly safe to patch the tire assuming as stated before from previous members the nail is not on the sidewall or close to it...another thing that you might come across at the tire centre is that if your tread is to low they can refuse to patch the tire because of safety...also if there was another patch in the tire before the puncture must be at least 90 degrees away from the other patch....if not they can refuse the the patch. any puncture in a tire ruins the structure of the tire and therefore it cannot perform aswell as it should...technically a patch is temporary. it just buys you a little more time to budget for a new one. you should only ride at 80km/h with a patched tire. it acts like a spare. but other than that it is perfectly safe and it will hold air pressure.. be sure to visit the tire shop asap, because if the tire is driven on a flat it is not repairable and you will be forced to buy a new one. assuming the tire is not run flat...