Our data suggest that the cannabinoid system is implicated in the psychoactive properties of caffeine and in the ability of caffeine to reduce the pathological consequences of stress. Chronic caffeine assumption sensitized GABAergic synapses to the presynaptic effect of cannabinoid CB1 receptor stimulation by exo- and endocannabinoids. The synaptic effects of caffeine were slowly reversible after its removal from the drinking solution. Furthermore, although exposure to caffeine for only 24h did not produce measurable changes of the sensitivity of cannabinoid CB1 receptors, it was able to contrast the down-regulation of CB1 receptor-mediated responses after social defeat stress.

I copied the rest from bloommedicinals.com, just the relevant bits. Also added the links to the studies and the pages with additional descriptions.

TL;DR:

Low levels of caffeine can help to maximize the effects of THC, but too much caffeine can lessen the impact of your cannabis

Caffeine and THC can both increase dopamine levels, but too much at once can trigger anxiety, agitation, and paranoia

What Happens When You Smoke Weed and Drink Coffee?

Since coffee and cannabis may offer so many health benefits on their own, you might think it’s safe to assume that enjoying a little bit of weed with your coffee would be okay. The reality is … it might be safe for most, but not for all. Here’s why.

THC and coffee can boost the amount of dopamine in our systems. Although dopamine can promote feelings of happiness and euphoria, too much of it at once can trigger anxiety, agitation, and paranoia. In addition, both caffeine and THC can both increase your blood pressure and heart rate.

This is why many doctors advise that those with heart conditions and those at risk of heart arrhythmia or stroke, should not mix caffeine and THC. So, if you’re new to mixing coffee and cannabis, it’s important to consult your doctor to make sure it makes sense for you. If it does, begin with lower amounts of coffee and cannabis than you would normally consume on their own. Then if needed, you can increase your dosage slowly.

Back in 2014, a study was conducted to measure the effects of mixing caffeine and cannabis on squirrel monkeys. The results of this study were published in the Journal of Neuroscience. (also described here)

The monkeys were given the ability to self administer cannabis whenever they wanted. All they had to do was to pull on a lever. When the monkeys were given low doses of a substance similar to caffeine, they seemed less likely to reach for the THC. But, when they received high doses of caffeine, they seemed to reach for that lever more often.

Then in 2018, a study led by Marilyn Cornelis at Northwestern University’s Feinberg School of Medicine seemed to back up these findings. This time, the study examined the effects of mixing cannabis and coffee on humans. It found that the more caffeine people consumed, the less cannabis their body was able to metabolize. (also described here)

So, although small amounts of caffeine can help your THC go a long way – too much caffeine can lessen its effects.