Yes you can.  In fact, I recommend using a bike you are quite comfortable on for your first few events.  As the comfort level rises, you can make your road bike more race ready by using slick tires that are 23-29 mm wide.  You can swap your platform pedals for proper racing pedals that you are connected to like a ski binding to your racing shoes offering you a much more efficient pedal stroke.  You can also add aerobars to your existing handlebars to get your body into a position with much less wind-resistance.  If you stick with the sport, and can afford it, you can eventually upgrade to a proper triathlon-specific bike later on.

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