Easily-confused!
If your specimen is a:and has stem­/­leaf arrangement:and the leaves have/are: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 (.pdf) Invasive > in Michigan inv. trading card (.pdf)

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]  "Direct Effects of an Invasive European Buckthorn Metabolite on Embryo Survival and Development in Xenopus laevis and Pseudacris triseriata."  Journal of Herpetology.  2014.  Accessed 2016-05-22.

Learn more about common buckthorn Rhamnus cathartica

Best Control Practices BCP (Michigan) (.pdf) Best Control Practices BCP (Ontario) (.pdf) and its Technical Bulletin (.pdf) Fact sheet (Ontario) 🇫🇷 en français (.pdf)

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