"With growing numbers of treatments that work when taken, the gap between the potential benefits of current treatments and the actual benefits continues to grow," Dr. R. Brian Haynes of the McMaster University Faculty of Health Sciences in Hamilton said in a statement.

The review, published in The Cochrane Library, found few people take all of their medications and many don't bother to take any.

The researchers looked at studies that were randomized, controlled trials that noted both adherence and outcome. They did find that a few strategies seemed to help -- including counseling, providing written instructions and pill packaging like the day-by-day dose used for birth control pills.

Haynes says doctors need to emphasize the importance of taking drugs properly and try to spend more time with patients who fail to do so.

The review found that almost all of the more successful long-term strategies were complex "combinations of more convenient care, information, reminders, self-monitoring, reinforcement, counseling, family therapy, psychological therapy, crisis intervention, manual telephone follow-up and supportive care."