- "Zoom never stole your speed. This is what's making you sick. Velocity-6 is killing you."
- —Dr. Caitlin Snow realizing the cause of "Jay Garrick's" disease[src]
Velocity is a series of drugs that allows temporary access to the Speed Force. Initially developed by Hunter Zolomon and then by Dr. Harry Wells as Velocity 6, the current version of the drug (Velocity 9) was created by Dr. Caitlin Snow. Despite its benefits, this drug is also very dangerous, as it causes cellular degeneration and eventually death. In addition, subjects of the serum have demonstrated mental instability or disorder, such as the split personality Eliza Harmon developed.
Wells first started to develop a speed-enhancing drug after Zoom appeared on his Earth, and it was meant to aid "Jay Garrick" in his struggles against the super-villain Zoom. However, the first five attempts (Velocity 1 to 5) were failures. A version of the drug, created by "Jay", seemingly caused the loss of his powers, as well as his cellular degenerative disease. In reality, "Jay" had in fact increased his speed, but at the cost of cell degeneration, forcing him to seek new, untainted sources of the Speed Force.
The drug was first mentioned by Harrison "Harry" Wells after he had traveled to Earth One and joined Team Flash. He explained its history to Caitlin Snow. She helped him create a successful version of the drug with her expertise in biochemistry. They produced the sixth version of the drug by adding sodium chlorate, as the drug needed an extra release of oxygen to function.
Though initially appearing reluctant to use the Velocity 6, "Jay" took the drug in order to save Wells after he had been shot by Patty Spivot. When Wells later thanked "Jay" for saving his life, the speedster however told him to keep the drug away from Barry.
When Barry had traveled to Earth-Two, Central City was attacked by Geomancer, and "Jay Garrick" had to take Velocity 7 to stop him. Due to the new serum's short-lived effects, Caitlin made Velocity 8, but was forced to update the formula due to the problems with Velocity 8. Velocity 9 was the first version of the compound to seemingly cause no side-effects, and it also triggered "Jay"'s latent regenerative abilities. Velocity 9 lasted long enough for "Jay" to save the day. 
During Velocity 9's development, Caitlin had asked Eliza Harmon for help, as she was having trouble with the synthesis. She sent information on 3 of the 8 components to Eliza. Eliza reverse engineered the drug, and used it to gain super-speed. The drug caused Eliza to develop an immoral split personality which became Trajectory. At Star Labs, Trajectory held Caitlin, Cisco, Harrison and Jesse hostage in order to obtain more Velocity 9. She injected Jesse Wells with Velocity 9 when she grew impatient, and Dr. Wells saved her life by undergoing a blood transfusion. 
Before Trajectory disappeared (possibly due to the accelerated cellular degeneration), her lightning turned blue. This, coupled with knowledge about "Jay's" illness, led the team to deduce that Zoom had taken the Velocity drug and was dying, leading to the discovery of Zoom's secret identity as Hunter Zolomon, who masqueraded as both Zoom and "Jay Garrick" all along to increase and then steal Barry Allen's speed, curing himself from cell degeneration.
CharacteristicsThe drug temporarily increases the user's speed beyond normal limits, and this speed far exceeds the speed of the Flash at Mach 2.2. The drug has several side-effects. A highly addictive drug, it causes rapid cellular degeneration. In one case, the user developed a psychotic second personality that caused her to commit criminal acts. At some point, the drug causes the lightning generated by a speedster to turn from the usual gold into blue. Also, as shown with Hunter, users of this drug can make their voice sound monstrous and can turn their eyes black.
Certain versions of the drug had more problems. "Jay Garrick's" version of Velocity 6 caused him to lose his connection to the Speed Force, and the cellular degeneration he had suffered was slowly killing him. Velocity 7 granted super-speed for too short a time. Velocity 8 had numerous kinks, which led to the immediate development of Velocity 9. While Velocity 9 initially seemed to cure the damage caused by the previous versions by also amplifying one's healing powers, it only delayed the symptoms caused by Velocity 6.
Because Zoom lived a long time with blue lighting, but Eliza lived for only a few seconds with blue lighting, a natural connection to the Speed Force seems to be able to slow down cellular deterioration.
- Hunter Zolomon/The Flash/Zoom (formerly; deceased)
- Eliza Harmon/Trajectory (deceased)
- Jesse Wells (unwillingly; formerly)
Behind the scenes
- In the DC comics, Velocity 9 was a drug created by Vandal Savage that granted a temporary speed enhancement. It was sold in the illegal drug trade. It too had severe side-effects.