There are some items you definitely want to pack for your trip to Thailand…

CONDOMS
You can buy condoms at any 7-eleven, drug store, or grocery here in Thailand.  However, it may take some experimentation to find the right product.  Like clothing, condoms are sized relative to the small build of the local population.  Thus, n XL shirt will probably be too small, and standard sized condoms may not fit comfortably past the head of your cock.  The last thing you want is to be struggling with a poor fitting condom in the heat of the moment.  Best is to bring plenty of condoms you know will fit, and then experiment with the local brands when more convenient.

UPDATE
The only safe brand to purchase and use in Thailand is Durex, available at any 7 eleven.  The largest is Excita, packaged in black.  Other brands have a tendency to break easily.

LUBE
While I’m sure you can find other products, if you look long enough, the standard in Thailand is KY.  It’s in every 7 Eleven, and drug store.  I have not gone looking for other options yet, because I brought a 17 ounce bottle of ID lube with me.  I strongly advise bringing what you prefer, as you won’t want to waste time hunting for an alternative to KY.

UPDATE
Pharmacies do carry other brands of lube.  I don’t recall the names.

IMODIUM
Diarrhea is the most common travelers’ malady, affecting 20 to 50 percent of Americans who travel abroad.  Thailand, with its poor sanitation conditions and spicy foods, is no exception.  The odds of you getting a mild case of diarrhea, within the first week, are very high.  Imodium can be purchased at any pharmacy.  Expect to pay about 60 Baht for 10 or 15 pills.

PEPTO BISMOL
Diarrhea aside, the spicy dishes, common in Thai cuisine, can give you an upset stomach or hear burn.  I’m sure there is a local equivalent here, but I’ve not found it yet.  I really wish I had packed a large bottle of Pepto, and you will be glad you did.

UPDATE
There are several liquid remedies available in Pharmacies, all similar to milk of magnesia.  One pharmacist told me the best is ZIGA.  Some pharmacies carry a couple of different antacid tablets.  The chewable tab is called MAG 77 (10 pills for 6 Baht), and the swallow only tab is GACIDA (10 pills for 10 Baht).  Of all three, I found the GACIDA was most affective for relieving heart burn.

TYLENOL and ASPIRIN
Tylenol and Aspirin are available here in Thailand.  You can get them at any 7-eleven, or drug store.  If your preference is for a different product, such as Advil, you’re going to want to pack it.

ANTIFUNGAL
The hot humid climate of Thailand makes for the perfect fungus breeding ground - both athlete’s foot and jock itch. You can get better remedies from the local drug stores, but they are not out on a display shelf.  You have to ask for them, and explain the exact area of infection.  I did think to bring an over the counter product, but it had little affect on the nasty case of jock itch I contracted early in my stay.  I still recommend packing something you prefer, as it will at least keep things under control until you can see a local pharmacist for something stronger. Every pharmacist I have encountered so far has spoken enough English to understand my needs and prescribe to proper treatment.

UPDATE
Most Thais seem to prefer ZEMA.  There is a liquid for treating feet, and cream for other areas.  Canesten is another cream, with identical ingredients found in ZEMA. All are very affective at clearing up a fungus infection.

COLD MEDICINE
You might not expect to catch the common cold while visiting Thailand, but I did.  Fortunately I brought a small supply of cold medication with me.  It was not enough to get me through, but I was glad I had it, because I happened to be in Laos at the time.  Again, the local pharmacy carries a fabulous remedy.  One tiny pill clears all the symptoms for 12 hours, and it does not make you drowsy. They have some really good drugs in this country!

UPDATE
Tylenol Cold is now available in 7 elevens.