Exploiting isomerism of molecules is particularly important in medicine or biological applications because enzymes tend to interact differently from one isomer to another.
In chemistry, two molecules are isomers if their molecular formula is the same, but they differ for the spatial orientation or the bond location within the atoms. In the first case we talk about stereoisomers while in the second case the molecules are defined structural or constitutional isomers. 
It should be noticed that “isomerism” doesn’t mean that the molecules have the same chemical or physical properties: in nature there are many examples of molecules that can have completely different interaction with biological targets even for a small difference in the position of a single chemical bond.
Phytocannabinoids derived from cannabis such as cannabidiol (CBD) and tetrahydrocannabinol (THC) could be taken as example to explain isomerism. These two compounds share the same molecular structure: 21 atoms of carbon, 30 of hydrogen and two of oxygen. Nevertheless the arrangement of these atoms is slightly different and this structural diversity is responsible for huge changes in the biological action on our body: both molecules interact with cannabinoid receptors (CB) of our endocannabinoid system (ECS), but while THC is psychoactive and binds mainly to CB1 in the brain producing psychotropic effects, CBD interact differently from the counterpart and it is capable of modulating THC activity on CB receptors. So the similarity of their structure make CBD and THC both lipophilic and capable of crossing the blood brain barrier (BBB), while the differences affect their interaction within receptors thus their therapeutic effects.
When THC is synthesized from CBD thorugh intramolecular cyclization it is possible to obtain mainly two different molecules depending on how the ring fusion will occur: THC-9 or THC-8. 
The former is the most well known kind of THC occurring also as major psychotropic substance produced by certain cannabis varieties, while the latter occurs only in traces in nature and differs from THC-9 just in the position of one double bond in the cyclohexene ring. The structural difference makes THC-8 less potent than THC-9 because of weaker interaction within the cannabinoid receptors, but it is also more chemically stable so less prone to oxidation or conversion to cannabinol (CBN) over time. In addition to THC-8, there are also other isomers that can be present in trace amount in cannabis plant such as THC-10 among others. The great challenge is to rationalize the differences within molecules and to study sistematically the subtle changes in molecular activity derived from the isomeric relation, that could lead to improved therapeutic benefits or reduction of unwanted side effects.
 Chhabra, Naveen; Aseri, MadanL; Padmanabhan, Deepak (2013). A review of drug isomerism and its significance. International Journal of Applied and Basic Medical Research, 3(1), 16–. doi:10.4103/2229-516x.112233 [Times cited = 175 ]
 Paola Marzullo, Francesca Foschi, Davide Andrea Coppini, Fabiola Fanchini, Lucia Magnani, Selina Rusconi, Marcello Luzzani, and Daniele Passarella, Journal of Natural Products 2020 83 (10), 2894-2901; DOI: 10.1021/acs.jnatprod.0c00436 [Journal impact factor = 4.803] [Times cited = 13]