Easily-confused!
If your specimen is a:and has stem­/­leaf ar­range­ment:and the leaves have:and the bark is/has:and the twigs:and the fruit (if present):then you have:
no more than waist-high (<1 m) (<3 ft)╟ alternate ▲▲ teeth no hair lookalike alderleaf buckthorn Rhamnus alnifolia
tree▲▲ teeth, visible within arm's length dark, lots of light-colored lenticels ;
if mature, looks covered with burnt potato chips­/­crisps
are small, black, taste like 🍒︎ cherry even if sour lookalike 🍒︎ black cherry Prunus serotina
___ smooth or
very small ▲▲ teeth (use 🔎︎ hand-lens)
dark, lots of light-colored lenticels no terminal thorn ;
hairy
lookalike glossy buckthorn Frangula alnus
sub-opposite small ▲▲ teeth dark, lots of light-colored lenticels ;
inner bark=­or­ange (scrape with knife)
often have a terminal thorn are small, black, taste dis­gust­ing after a few seconds—spit it out this plant, please continue below . . .
╫ opposite (most species) ___ smooth,
taper at both ends, and
pass the dogwood leaf test
no con­spic­u­ous lenticels nor strong colors lookalike dogwood genus Cornus

Verify with Google images.

Also called purging buckthorn.

Native to 🇪🇺 Europe, northern Africa and western Asia.

Buckthorn leaves, fruit, ❀ flowers, bark and roots produce and release the allelopathic chemical emodin , which has been shown to kill or malform amphibian embryos, and immunosuppress and produce abortive effects in birds and mammals.  Areas in which this species is invasive have been shown to suffer a decline amphibian diversity and abundance, including of our native western chorus frog Pseudacris triseriata .[1]   Invasive > learn+quiz Invasive > Global Invasive > in USA+Canada Invasive > report it! Invasive > in USFS Invasive > in Michigan inv. trading card

Uses by native peoples
(Ethnobotany database)

References
[1] http://cnah.org/pdf/88516.pdf retrieved 2016-05-22 by EP.

Learn more about common buckthorn Rhamnus cathartica

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