Jogging along the Santa Monica beach, a traveler twisted his ankle. He went to a clinic where an X-ray revealed no fracture. That was two days ago, he informed me, and the ankle was healing. He was to fly home tomorrow, and his doctor in Switzerland had urged him to get an injection to prevent a blood clot. I've received a sprinkling of these requests, all from foreign travelers. They began a few years ago when the media began reporting clots in travelers after long plane flights. The risk in healthy people is tiny and concentrated among those who fly more than four hours, but it's not zero.

Drugs to thin the blood such as Coumadin and heparin have been around for decades but are too dangerous for healthy people. In 1993, the FDA approved Lovenox, a refined form of heparin, safe enough for use outside a hospital. My wife gave me six weeks of daily injections after I broke my hip in 2003. It was still under patent and wildly expensive, but generics have appeared, so it may soon become fashionable to get a shot before a long flight.

So far no Americans have mentioned the subject, and I give foreigners the traditional advice: walk around and drink plenty of fluids. Techniques that don't work include compression stockings (unless fitted by a professional they make things worse) and taking aspirin. Aspirin prevents clots in arteries, but prolonged immobility affects veins.

Mike Oppenheim