People may be tempted not to take their meds when they feel well, and end up doing so only if symptoms come back. This behavior greatly increases an individual's chances of developing resistance to his or her meds. Once someone goes off medication, the viral load can rise substantially and there would no longer be anything to limit virus reproduction. When one has a high viral load, there is a great risk that damage to the immune system will occur. If a cocktail is discontinued, it is recommended that all the drugs in that cocktail be stopped, rather than just one to two drugs.
Drug resistance can limit some treatment options for the future.
Sometimes, a person may become resistant to multiple drugs at the same time, further limiting their future treatment options. No level of adherence can guarantee that one's virus won't develop resistance. The most realistic approach is to know that the more adherent one is, the better one's chances that the medications will work.