what are the consequences of drug resistance in HIV patients?

It depends on the level of drug resistance, which can be minor, major or somewhere in between. In general though, acquiring drug resistance means that the response to the drug or drugs to which your virus shows resistance will not be as good as the response to the same drug in someone who does not have resistance. In other words, resistance is another way of saying that a virus has developed the ability to replicate (or reproduce) in the presence of the drug that is supposed to be blocking or inhibiting its replication - this leads to treatment failure in the worst cases and suboptimal treatment responses in the remainder. The difficulty is in defining the degree of resistance. Someone who is familiar with reading drug resistance test results can give you a fairly good idea if you carry any of these resistance mutations or if not, how certain he/she is that this means you do not really have any present. There are unfortunately times during which drug resistance cannot be detected (for instance when the viral load is very low - say less than 500 copies), though resistance mutations may still be present - perhaps to drugs in previous regimens though and not the drugs in the current treatment regimen.