Does shaving your hair take away the trace of drugs? Or when the new hair comes in does it carry the same trace?

Can they shave your eye brows for a hair follicle test?

That's an interesting question.

First, it would depend on the specific drug and when you last used. It is estimated that drugs that are incorporated into the hair from the hair bulb take 3-7 days to appear in the hair shaft above the scalp*. Of course that depends on how fast your hair grows and growth rates vary by a factor of 3x.

However, if you had used drugs within a couple of days of shaving your head then there could be some of the drug in your hair that hasn't made it to the surface yet*. But even that is dependent of the drug. In that case, certain drugs would be detectable as the hair grows back in proportion to the amount used during that time.

Shaving your head is often a self-defeating action. Collectors are trained to obtain the hair from an alternative source such as chest, axillary (under arm), pubic, leg. These alternative sites, having different growth rates, often contain drug at far greater concentrations than head hair and retain it for a longer period.

Point of interest: Hair drug testing never tests the hair follicle. Only the visible hair shaft.

* Illustration of human hair showing range where drug could exist, between dermal papilla and emergence above scalp.

As ex: Research and Development in Forensic Toxicology