While planning a vacation, your mind swims with images of tropical shores, ancient ruins, exotic markets -- not of you heaving over a foreign toilet or tossing, feverish, in an unfamiliar hotel bed. But sick happens, and while some travelers pooh-pooh the notion of packing a mobile medical kit, for many people just one ugly episode is enough to spur stashing a few supplies.

So what should you stash, anyway? During a recent Travel section online chat at www.washingtonpost.com, we asked readers what sorts of medication they pack when on a trip. Their answers were both enlightening and startling (see accompanying story). Then we went to more authoritative sources, including travel-health experts and travel writers, for ideas on what to stow in your little black bag.

Christie Reed heads the Travelers' Health team at the U.S. Centers for Disease Control and Prevention (CDC), which produces a publication called "The Yellow Book: Health Information for International Travel." Reed, a physician specializing in epidemiology, internal medicine and pediatrics and a world traveler, noted that her advice echoes that in the book's second chapter. You can access the entire text online at www.cdc.gov (click on "Travelers' Health," then on "References and Resources").

Reed recommends that those traveling to exotic foreign locales start with a visit to a health clinic at least four to six weeks before starting their trip; go to the CDC Web site (click on "Travelers' Health," then "Travel Medicine Clinics") to locate one near you. Travel health physicians can help pinpoint which, if any, vaccinations you'll need, depending on where you're headed. (Hence the long lead time: Some vaccines take several weeks, or second doses, to become fully effective.)

Clinic staff can also guide you in assembling a travel medicine kit appropriate to your destination; a trip to an underdeveloped country might warrant a bigger supply of an antidiarrheal such as Imodium than a trip to, say, London.

"Happy travelers think ahead," Reed said. Other suggestions:

* Imodium tops Reed's (and just about everybody else's) list of must-pack travel medications, as unfamiliar food and water can trigger diarrhea -- if only for the first few days -- even though it leaves the locals unaffected. If you're embarking on a long-term adventure in an underdeveloped land, Reed says, you should consider bringing iodine tablets to purify water.

* Pack any prescription drugs you regularly take in their original containers (with prescription labels) in your carry-on bag. Include extras in case your itinerary changes. It can't hurt to also carry a letter from your doctor saying what meds you take and why you take them, especially if you're taking injectable drugs and need to carry syringes onboard.

* Get a doctor to prescribe a small supply of antibiotics (such as Cipro), just in case you contract a bacterial illness (including diarrhea); if you don't use them on this trip, Reed said, you can keep them in your kit for next time.

* Include any over-the-counter drugs you typically use at home; you might not be able to find satisfactory equivalents where you're going. This is especially important if you're traveling with kids, Reed noted. "Bring all the basic things you'd want to have in the household if somebody had a problem in the middle of the night," she said. Make sure to pack a fluid and electrolyte replacer such as Pedialyte in case a child gets diarrhea, which can dehydrate little ones quickly.

* When visiting an area where malaria or dengue occur, carry a mosquito repellent containing 30 to 50 percent DEET.

Infectious disease specialist Phyllis E. Kozarsky, travel health consultant to the CDC and a professor of medicine at Atlanta's Emory University, added to the list. She recommended taking ibuprofen (Advil, Motrin) or acetaminophen (Tylenol) for pain, headaches and mild fever; Dramamine for motion sickness; Benadryl for mild allergic reactions; a prescription sleep aid (such as Ambien) to prevent jet lag; and a decongestant.

If you don't feel up to assembling your own meds, Kozarsky -- who also is co-editor of the encyclopedic "Travel Medicine" (C.V. Mosby, $155) -- suggests you buy a prepacked kit, available at such sites as www.travmed.com. (Other sources, including one for adventure travelers, are listed on the CDC site under "Illness and Injury Abroad.") A basic kit from the Travmed site containing a travel-health guidebook and such standard first-aid supplies as bandages, antibiotic cream and tweezers costs $36.95.

As for the suggestions culled from the Travel's section's online chats, Kozarsky agreed that travelers should put a note in their passports regarding any drug allergies, heart condition, diabetes, etc., to alert first responders, and include your doctor's business card. And she seconded the chatters' admonitions to carry throat lozenges (for dry throat on the airplane); condoms; cortisone cream for insect bites; a small bottle of rubbing alcohol or iodine; and sunblock that protects against both UVA and UVB radiation.

While Reed and Kozarsky see travel medicine from a doctor's point of view, Tony Wheeler comes to the issue as one who's been there and lived to tell the tale. Wheeler and his wife, Maureen, are the founders of Lonely Planet, the travel guide series.

When asked what he packs before heading off to parts afar, Wheeler said in an e-mail, "I was about to say nothing special at all, I'm a real believer in letting things sort themselves out, I really don't believe in tossing down antibiotics at the first sign of a stomach upset. But then I thought of a few occasions where my medical kit has come into its own."

* In a remote part of Tibet, he came across a pair of other travelers who both looked sick. Wheeler, figuring they suffered from the intestinal ailment giardiasis, was able to treat them because he happened to have packed the antibacterial medication Flagyl.

* Stuck on an island in the Tuvalu group, Wheeler and the photographer he was traveling with got cuts from brushes with vegetation or coral. When the cuts became infected, Wheeler pulled out his store of the antibiotic erythromycin, and all was well.

* When a walk in Australia turned out to be much more rugged than he'd expected, Wheeler was "very glad" to have blister treatments on hand (moleskin and something to cut it with).

* A German brand of wound-closure strips (like the 3M-brand Steri-Strips sold in the United States) saved the day when a fellow traveler fell and got a cut above the eyebrow that was "just about bad enough that you'd have thought about stitches."

* When the Wheelers' daughter, then 16, got food poisoning in Guatemala, Wheeler said he and his wife poured the electrolyte-replenisher sold overseas as Gastrolyte (similar to Gatorade) into her mouth, trying to keep her hydrated. He added that the stuff often has come in handy.

Like Wheeler, Patricia Schultz started out with a claim that she doesn't tend to pack much in the way of meds when she travels. The author of "1,000 Places to See Before You Die" (Workman Publishing, $18.95) explained, "There are so many possible scenarios out there, how do you second-guess? I, resultingly, don't."

Her favorite bit of advice: "If you're visiting a developing nation, go right to the hotel [staff]. They know about things that befall Americans or foreign guests." Not only can hotel staff help protect you from potential health threats, Schultz said, they can help get care if you do take ill. "They have reliable, English-speaking doctors on speed dial," Schultz said.

Schultz, who admitted that she winds up using Imodium "more than I like to think," said her strongest weapon is simple in principle but hard to implement: "I'm extremely careful about what I put in my mouth. What befalls you is mostly about what you eat."

Those seeking more information can check the World Health Organization's Web site at www.who.int/ith/enA. Many universities sponsor travel health clinics; check the University of Maryland's site at www.umm.edu/travel/guide.htm.

Jennifer Huget last wrote for Travel on family cruises.