Don't do it.
1) It's insanely, incredibly dangerous because it's so difficult to gauge an accurate dose. The OD potential is phenominal
2) It's difficult to break down. If you MUST do it, a tiny amount of a weak acid (like citric acid) MUST be added to convert the freebase fentanyl in the gel into a water soluble salt. Without doing this, most of the fent will just stay in a blob of gel and won't go into the aqueous solution.
3) Even if you do sucessfully prepare a solution using the weak acid that can be IV'd, some of the polymers in the gel will wind up in that rig, and I don't even wanna think about how terrible it is for your body to put that crap in your circulatory system.
Smoke the gel using tiny drops at a time (this method is actually relatively safe, since it hits immidiately so doing a little at a time is OK, and you can guage the dose well because of that). However, fentanyl laden gel smoking can get VERY dangerous indeed if self control goes out the window -- smoked fentanyl is the crack/freebase cocaine of opioids. You get a rush, a short high, then fiend for another hit like crazy. It's a good way to blow through your stash way too quickly.
Rub the gel (small drops at a time) on your cheeks, gums, and under the tongue evenly and wait awhile (the time varies person to person). Repeat if you desire a stronger high. Don't swallow before 30 mins have passed or you're wasting a lot of fentanyl. Take a cyp3a4 inhibitor like grapefruit so any fent you do swallow has a shot at being absorbed (but if you do take a 3a4 inhibitor, be even MORE dose conscious because OD potential is increased up another notch). BE CAREFUL... this is actually risker than smoking, OD wise, since it takes a bit to kick in, and a redose too soon that is too large can easily make you fall out. This method is, IMO, a PITA, since absorbtion is only optimal as long as the fentanyl gels stays affixed to your mucous membranes (spit production makes this impossible after 10 minutes... but once that happens, keep swishing the spit around anyway... more will still absorb).
I personally (considering the buccal route only) get higher on the same mcg per hour mylan patch than I do a gel patch. I think it's because the mylan sits RIGHT up against the moist mucous membrane and releases only a limited amount at a time, allowing for MOST of the fent to easily absorb into the blood vessels in the mouth, while with gel, spit builds up and takes up the fent, moving it away from the cheeks and gums (the mylan prevents the spit issue by being right up against the cheek which keeps most spit out of the way and gives the fent contents a straight shot into the capillaries behind the cheek). However, the mylans release fast enough cheeked (buccally) to get you high -- IME mylans release all or most of their fentanyl content in about 6 to 8 hours buccally (vs in 72 hours when worn as prescribed on the skin) -- clearly that's quick enough to get one buzzing. Some people seem to prefer the gels even for buccal administration though; I seem to be an oddball in this regard. I definitly am not experiencing any placebo effect though.... I get higher from buccal mylan of the same dose patch than I do with buccal gel from a gel patch (of course, with the gel, you have to space out the blobs of gel over many hours... rubbing the whole patch's contents onto your cheeks and gums and once is asking to either OD or waste a whole lot of fent by swallowing it). Keep your tolerance in mind for buccal gel... you'll figure out how much of a blob you'll need at once to get buzzing, but start slow.
Any of these options is so much smarter than trying to IV fentanyl from gel patches. Even if you managed an extraction, how would you know what the yield was (how much was actually extracted)? If you don't know... it's awful hard to dose accurately. It's pretty much impossible to dose unextracted gel accurately by IV, and even if you do it and don't die, you're putting some NASTY stuff in your veins... gelly polymers in veins... doesn't sound good does it? It'd be tough (no, impossible actually, without a well stocked lab and chemistry experience) to tell how much fent you got out of an extraction.
If you really want to use fentanyl intraveinously, get ampoules of fentanyl citrate meant for injection (best way) OR get fentora buccal pellets, cut the pill with a pill cutter to your desired dose (no more than 200 ug for a first try... 100 ug is even better for a test shot), and break it down in a spoon. Wait for the fizzing reaction to stop... this is two additives in the pill reacting... they're there to help the pill dissolve buccaly when used as prescribed. The additives are definitely a carbonate and a weak acid -- if you wait for the fizzing to stop, the salt compound that results will be water soluble and end up in your shot, but it'll be pretty harmless. Filter (wheel filter preferably) and then shoot up. If you must shoot a NON ampoule fentanyl product, Fentora is the ONLY remotely safe option. Keep in mind that fentora is very effective at getting you high when used as prescribed too (buccally). They don't get a person on a regimen of them daily for b/t pain high (unless they take a higher than usual dose), but they'd get a recreational user high that way. Either way, again, start slowly and work up till you find the right dose to get you there. Keep in mind that IV fentanyl is likely to have a short duration just like smoked fentanyl freebase in the gel. Duration is about an hour smoked with occasional use IME, but can shorten down to 10 to 20 minutes during a heavy binge (and that's when it becomes dangerous, since fent only has a short duration via this ROA because it's taken up into fatty tissue... it's half life is actually similar to other common Rx opioids. therefore, frequent redosing, crack style, can easily lead to an OD if you do something, like exercise, that releases some of the fent hiding out in your body fat. It's also easy to OD if you begin to chase the rush and not just the buzz). I believe all of this probably applies to IV fentanyl too.
Please be careful. I STRONGLY urge you to NOT shoot up broken down gel from duragesic patches. Please. You run a high risk of DEATH.
As an aside, the only way I enjoy recreational fentanyl anymore is buccally, preferably with a mylan patch (sometimes while wearing another low dose patch.... but this is another "with caution" thing... I know my tolerance. I'd never recommend wearing a patch as you're using another ROA at the same time to ANYBODY. I only do it because I know how to do it safely from experience). My second choice would be using gel buccally. I never smoke freebase fentanyl laden gel anymore because the high is just too short, and it gets very crackhead like. Smoking fentanyl makes your behavior scream "I'm a drug feind" to everyone around you.
Recently I've only been wearing as prescribed anyway. I'm kinda proud that I've been being so good and faithful to my maintenance and super slow taper regimen. It's not so bad really, fentanyl maintenance kills cravings really well. But the above paragraph is how I prefer my recreational fent at the times I used it recreationally, and that's how I'd do it if I decided to treat myself one day.
My closing statements are: Fentanyl is a tricky drug. It lacks the versatility of other Rx opioids with regard to ROA options. Fentora pellets are the best if you can't get amps but want to IV. Plugging gel or fentora solution may work well, but if it's gel, you run into the dosing difficulty again (but it's MUCH safer than IV gel!). If you want a high akin to a high from IR opioid pills,matrix patches or gel from resivoir patches buccaly, or fentora/actiq as its meant to be used but in a dose that will get you high, are the best options. If you want a high like IV heroin or IV pharm pills, your options are more limited: 1)smoke the gel but be careful... try not to get crackhead-y, don't overdo it by chasing the rush... smoked fentanyl freebase has a better rush than smoked heroin since you can get a FULL dose in one hit if you're skilled at puffing fent, but with heroin you can't get a high enough dose in one hit for a good rush... fentanyl's potency allows for you to get rushes from smoking ROA
2)Prep fentora pellets for IV use. Use a pill cutter to cut a piece off that's the desired doseage. Wait for fizzing to stop, than proceed like any other IV of a pill. Using a wheel filter would be absolutely preferable; your body will thank you.
No matter what you decide to do, keep these harm reduction tips in mind. It's easy to fall out before you can even say "fuck" and just die of OD with fentanyl. You've gotta exercise even more caution than usual with fentanyl than other opioids.
Enjoy your high