You'll find pharmacies in the major cities. Pharmacists in Vientiane and Louang Phabang are quite knowledgeable and have a decent supply of medicines.

Health care in Laos is so poor as to be virtually nonexistent. The nearest medical care of any competence is in neighbouring Thailand, and if you find yourself afflicted by anything more serious than travellers' diarrhoea, it's best to head for the closest Thai border crossing and check into a hospital. A clinic attached to the Australian embassy in Vientiane is mainly for embassy personnel, but can be relied upon in extreme emergencies.