Distillation of tetrahydrocannabinol (THC) leads to one of the most potent cannabis products on the market. However, pure THC distillate lacks all but THC. In other words, it’s missing many but not all of the desirable characteristics of cannabis plants, such as taste and smell. Fortunately, distillate manufacturers and even consumers can remedy this by adding back to the distillate what was lost.
Terpenes and THC Distillation
Cannabis terpenes are chemical compounds that produce the smells and tastes consumers find pleasing. They also have powerful medicinal properties and have attracted the attention of the pharmaceutical and health industries for their medical potential.
However, distillation performed at 157 °C removes these and many other compounds, including other cannabinoids, leaving a specimen of essentially pure THC. The product is highly potent as you would imagine. And it can be consumed like other extracts. But it lacks flavor and smell.
THC distillate also lacks other beneficial effects found in cannabis products containing terpenes. For example, the entourage effect refers to the effects consumers feel when terpenes, flavonoids, and cannabinoids act together.
Much research needs to be done on the entourage effect, but it is a verified phenomenon with cannabis, and current research suggests it may play a powerful role in treating health issues. 
Adding Terpenes Back to THC Distillate
As you might imagine, the industry has come up with a way to add lost terpenes back to distilled THC. Consumers can now enjoy the potency that distillates have to offer without sacrificing the benefits that terpenes bring. Even more, distillate manufacturers can control the terpene profiles of their distillate products and create a diverse line of distillate products.
So how is it done? By simply selecting the desired terpenes and adding them to the distillate. Depending on the manufacturer, the added terpenes may constitute as little as 1% or less of the THC distillate, upwards of 10% or more. Consumers can also do this with THC distillate oils they have. A wide array of terpene products are available for purchase, even those not originating from cannabis plants.
Some manufacturers and consumers prefer to keep their distillates pure, typically because they use them to make other cannabis products, such as edibles. That said, they can also benefit from adding terpenes back to their distillates.
Ferber, Sari, G. The “Entourage Effect”: Terpenes Coupled with Cannabinoids for the Treatment of Mood Disorders and Anxiety Disorders. Curr Neuropharmacol. 2020;18(2): 87-96. doi: 10.2174/1570159X17666190903103923. [Journal impact factor = 7.363] [Times cited =95]