When it succeeds, the drug may no longer work. Improper use of a drug hastens this process. Sometimes mutations occur naturally, even before a person has ever taken a drug. Sometimes a person was infected by someone who already had the resistant mutation. Improper use of a drug Failure to take a drug as prescribed is probably the most common cause of drug failure.
Improper use speeds the development of resistance.
Use the therapies as prescribed. If this fails, try new therapies and use them exactly as directed. Inadequate absorption or metabolism of a drug This may be common in people with wasting syndrome and malabsorption problems. If the gastrointestinal system can't process foods properly, it probably won't do a very good job with drugs either. Sometimes increased doses of the drugs will help, though this risks greater toxicity.