You cannot compete without one. This is not a country where you can sign a liability waiver and do any harebrained thing you want, like in the US. I remember signing like 12 documents about how if I died or injured myself, I wouldn't sue this skydiving company (whose office was what seemed to be an abandoned airplane hangar in a cornfield, not particularly confidence-inspiring) for taking me on a tandem jump, nor would anyone in my family ever sue them, because it was my own crazy idea.
On the one hand, I am all for you being forced to reveal your tri or running goals to your doctor and have a chat about health in general, your training and what kind of shape you're in. It could be a good way to start a dialogue, get some idea of your baseline fitness and better safe than sorry, why not have a little physical checkup at only minor personal inconvenience, like going there and paying 23 euros, 75 percent of which is reimbursed by the French government? People always tell you about the Perfectly Healthy Runner Who Dropped Dead Of A Freak Heart Attack During the Race, don't they? I've heard that one several times. Why not get a quick physical? Ounce of prevention and all that.
On the other hand, it's also kind of a pointless farcical waste of time. The certificate is carefully worded to say that the doctor "didn't find anything today that could prevent you from competing in a race or a triathlon." In French, it's called a "certificat de non-contre indication." It mainly says, in lots of clever French negatives, you don't seem not to be ok-- and if I'm wrong, the doctor might as well add, because this is France and not the US, I probably don't need to worry about being sued... The medical certificate is valid for a year.
I went from being horrified by the French cultural experience of going to the doctor to actually enjoyng it. My doctor is easily 400 years old and sometimes if you call for an appointment, her feeble, elderly voice will come on the answering machine saying that she's away for a few days in the countryside resting. She examines patients more or less in her living room, on a folded up sofa bed futon with medical exam paper on it. She also likes to scold me, but nicely, I just think this is how older French women interact with younger women. I was affectionately scolded last week for not having a current doliprane prescription on hand at all times and she promptly wrote me one stat to avert the possible crisis of being doliprane-less. Doliprane is the favorite French drug for everything from the flu to headaches and it's a pill popping culture. For the common cold, elderly doctor has prescribed up to 5 different meds for me to take all at once. I personally hate taking pills, even tylenol (or doliprane) for headaches, and will go to elaborate lengths to avoid it. I've been going to elderly doctor for a few years now, we get on well. Every year I mean to send her a Christmas card but never do. I no longer live in the same area as her office, but I really like her, so trek across the city a few times a year to see her.
Some doctors just sign your medical certificate. Not mine, she is pretty thorough and we chat about my training. She examines me and she listens to my heart and takes my blood pressure (she uses her ancient little alarm clock and waits for the second hand to sweep along the clock to see when a minute's up). Then we have a little ritual where I have to do 30 squats and she takes my blood pressure again immediately afterwards to monitor recovery time and she listens to my heart again. This time, I was complimented on my fitness, she said she could tell I exercised a lot and that I was in good shape.
Everything was ok and she kindly signed not only a running medical certificate but also a triathlon one (you can probably use the tri one for running, too, but I didn't want to take any chances). It was nice to see elderly doctor, but I'm still on the fence about whether or not these certificats médicaux are responsible sports medicine and important for athletes or just kind of pointless and another French bureaucratic hoop to jump through. And to jump through a hoop either in training or in competition in France, you would, of course, need a medical certificate!