Warning: spoilers are ahead for Episode 3 of Arrow Season 3, "Next of Kin."
Arrow made a big change in Season 6 when Oliver made the decision to pass on the legacy of the Green Arrow to his longtime partner and best bro John Diggle. There's no character in the Arrow-verse more deserving of taking on the mantle of the Emerald Archer of Star City than Diggle, but the timing was pretty unfortunate. Oliver passed on the hood just before Diggle was going to confess that he had degenerative nerve damage in his dominant hand. Only Dinah knew that Digg was compromised in the field.
In "Next of Kin," we saw Diggle hit the streets as the Green Arrow (although without the use of any arrows); by the end of the episode, it became clear that he was keeping an even bigger secret than we thought, and this is a secret that could spell disaster for Star City. Instead of admitting his nerve damage to the rest of the team, Diggle is buying black market drugs that are apparently enabling him to power past his tremor.
Arrow didn't specify just what exactly Diggle is buying and injecting himself with, but it's clearly enough to overcome the effects of the tremor that had kept him from firing his gun and having his teammates' backs in the field. On the one hand, it's great for Diggle that he has a way to regain his full strength and keep doing the work that he loves. On the other hand, it's unfortunate that he's apparently doing something very sketchy to make his recovery happen.
If this substance he's injecting himself with was something legal and/or safe, he probably wouldn't be doing it in secret. Besides, there must be side effects. A drug so miraculous that it even temporarily fixes the symptoms of degenerative nerve damage is simply too good to be true, and it wouldn't be sold out of black briefcases in seriously sketchy alleys if everything was on the on the up and up. The fact that he's convinced even Dinah that the tremor is no longer a factor means that Team Arrow likely feels a false sense of security in his abilities.
After all, Diggle seems to have an arrangement about picking up the meds from his supplier, and that wouldn't be the case if he didn't have to regularly dose himself. The drug clearly wears off; if it wears off at an inopportune moment when Digg isn't able to take another dose, lives could be lost. Throw in the fact that Oliver just set the legislative wheels in motion to put the legality of vigilantism to a city-wide referendum, and there's a lot at stake. If the Green Arrow starts to mess things up for the city because his tremor pops up at the wrong time, the entire team (and his family) could face the consequences. Add on the FBI investigation courtesy of Samanda Watson, and Diggle's life isn't going to get any easier the longer he keeps this secret from his teammates.