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 and 🇪🇺 European buckthorn.

Native to 🇪🇺 Europe, northern Africa and western Asia. Invasive > learn+quiz Invasive > Global Invasive > in USA+Canada Invasive > report it! Invasive > in USA Invasive > in USFS Invasive > in Michigan inv. trading card

Uses by native peoples
(Ethnobotany database)

On a list as having seeds and leaves mildly poisonous for people and animals, but readily eaten by 🐦︎ birds.

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]  

References

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

Learn more about common buckthorn Rhamnus cathartica

Best Control Practices BCP (Michigan) Best Control Practices BCP (Ontario) and its Technical Bulletin Fact sheet (Ontario) 🇫🇷 en français Discover Life Encyclopedia of Life Google Google images Michigan Flora USDA PLANTS db USFS Wikipedia