Parts of Laos have a good deal of Malaria so anti-malarials are recommended if visiting those areas for an extended period, but check with health professionals: there is a high incidence of drug-resistant parasites in these parts. Other mosquito-born diseases, such as dengue, can be life-threatening, so make sure you bring at least 25% DEET insect repellent and ensure that you sleep with mosquito protection like nets or at least a fan. Vientiane seems to be malaria-free but not dengue-free. The mosquitoes that are active during the day carry dengue and those that are active in the evening carry malaria.
The usual precautions regarding food and water are wise. Bottled water is widely available.
Vientiane has several medical clinics associated with European embassies. Care in the clinics is up to First World standards. Otherwise, you probably have to go to Thailand for treatment of serious injuries and illnesses. Udon Thani and Chiang Mai are generally recommended; they're only a few hours away, depending on your location in Laos. Ubon Ratchathani and Chiang Rai might have suitable clinics, as well, and there's Bangkok, of course. Expatriates in Laos probably have the best information; the more upscale hotels can be good resources, as well. Medical travel insurance is a good idea.
- Crime levels are low in Laos, although petty theft (bag snatching) is not unknown and seems to be on the rise. It is highly unlikely to affect most travellers, but Laos has some of the world's highest corruption levels and is a big factor in many citizens' lives.
- Judicial processes remain arbitrary and, while you are unlikely to be hassled, if accused your legal rights may be slim or non-existent.
- Sexual relations between a Lao national and a foreigner are illegal unless they are married, and marriage requires special permits. Lao hotels are not permitted to allow a foreigner and Lao national in the same hotel room together. "Number One" condoms are available for 1000-5000 kip for a pack of 3. These are probably the cheapest condoms in the world (and their quality seems to be ok).
- Drugs are a large problem in Laos and should be avoided at all costs. Lao law makes little distinction between personal use and trafficking and any conviction will result in heavy fines and expulsion at best and imprisonment or even execution at worst. Methamphetamine is widespread and often offered in "special" or "happy" shakes along the backpacker trail.
- Criticism of the Lao government or the Communist Party in any way, shape or form is unwise.
- Landmines or unexploded ordinance left over from the Vietnam War maims or kills hundreds of people every year as Laos is the most bombed country in history. Almost all of these occur in the eastern and northern parts of the country, especially near the border with Vietnam. Never enter into areas marked as minefields and travel only on paved roads and well-worn paths. If you are unsure of which areas are safe, ask locals.