There is much debate about the pace at which body hair grows. Many people are concerned that an inch and a half of body hair will retain drug residue for a much longer period if the body hair grows much slower than the head hair.
The reality is that body hair and head hair grow at the same rate. The difference is that body hair tends to stop growing at a shorter maximum length. In both head and body, new hair replaces dormant strands and pushes them out.
Think about how you naturally shed hair. Shedding happens because a new hair pushed out the old one, which stopped growing and became dormant. When a hair is dormant (i.e., not growing), no new drug deposits can get into it. It can stay there 1 to 4 months, so in theory, head OR body hair can reveal drugs that go back further than 90 days, depending on the person. However, only 10 to 15 percent of your strands are in this dormant phase, so the risk is minimal.
On the other hand, there are many anecdotes (and a few labs) that report that body hair can pick up drugs for up to 12 months. Although there are no scientific studies that have shown this conclusively, you'd be wise to consider it when thinking about how to interpret the results.
The bottom line is: the 90-day mark is an educated estimate, but there is no calendar in your hair. Accordingly, a positive result doesn't indicate the dates that the drugs were used. In conclusion, your hair retains your drug use history, but not in the same way that, say, an internet browser retains your search history.