Yes the products which contain the glaucoma treating active ingredient in pharmaceutical level percentages probably do work. For patients with glaucoma the lash growth and darkening was an observed side effect (along with more serious issues like permanent changes in iris colour). But any cosmetic product you buy in the UK (like Rivatalash) is not allowed to contain the active ingredient in a percentage anywhere near pharmaceutical level. So the chances of any side effects (including lash growth and darkening - but also the nasties) are lower.