First, there are five primary causes of injection pain.
1) High Benzyl Alcohol content.
Benzyl Alcohol (BA) is used to increase the lipid solubility of esterfied compounds and to prevent bacteria growth in the oil. Most UGL's use excess BA or only BA as the co-solvent, due to it's modest price. Unfortunately, it can cause the destruction of cells and significant localised pain and inflammation. The discomfort is characterised by fairly rapid onset of a painful, red, swollen area. This can be ameliorated by diluting the compound with sterile cutting oil (grapeseed, cottonseed, sesameseed). A 1:1 ratio often works well.Benzyl Alcohol in high concentrations will destroy any cell. That's what gives it the antimicrobial properties that we take advantage of when compounding oil based steroids. Users can experience this as a hard red swelling in the muscle that usually goes away after a few days. By using more benzyl benzoate then BA you reduce the chance of this becoming a problem. If you hurt to the point where you feel like you need crutches, the UGL has not appropriately compounded your oil. Follow the directions below and you should be fine.
2) Precipitation of short estered compounds
The shorter an ester, the less solubility is has in oil. This necessitates higher concentrations of BA to prevent the compound from 'crashing' out of solution. When the oil is injected, the BA is absorbed from the location of the oil depot. This caused the active compound to precipitate fine crystals within the muscle. The onset of this pain is often 4-12 hours after injection and is characterised by a hard, red, swelling at the injection site. To prevent this an additional co-solvent is needed, Benzyl Benzoate. BB maintains solubility of the esterfied compound past the point at which BA has been removed, preventing crystaline precipitation. Most UGL's don't add this to their compounded oils due to the higher cost. If added at approximately 20% by volume you can prevent most associated short ester pain.
3) High concentration (mg/ml)
Per above, Long ester = high solubility, short ester = less solubility. Human grade pharmaceutical hormones are almost never dosed higher then 200mg/ml for long esters (cypionate). To increase the concentration, a larger amount of BA is necessary as a co-solvent. This can result in a similar reaction to that caused by short esters with pain and swelling as the BA is absorbed and solubility decreases and precipitation occurs. In this instance you don't 'need' the additional co-solvent (BB, but it still helps!). Instead, you can simply dilute the compounding oil with sterile cutting oil to lower the concentration to within the solubility for that ester.
4) Fascia displacement
This occurs when a large volumetric quantity is injected into a single site. The oil pushes the muscle fascia apart and results in stretching and eventually scarring with the muscle. This can be avoided by reducing single site injection volume and spreading the injections between multiple sites. This is also a good idea as more smaller, more frequent dosing significantly reduces many of the side effects of AAS. These become exacerbated by peak and troughs in hormone levels. The reduction in side effects is particularly dramatic with trenbolone acetate when an ED injection schedule is maintained.
5) Histamine reaction
This doesn't occur as often, but is still a potential possibility. For a compound to become biologically active the ester must be cleaved from the parent molecule. This results in the formation of a carboxylic acid. Some people find that they are sensitive to one of these. This results in swelling, warmth, redness and possibly a rash at the injection site. In the worst instance the individual may have an existing allergy to the carrier oil (sessame, etc...) that can result in anaphylaxis (swelling and occlusion of the airway) in addition to localised inflammation. If any histamine reaction results, immediately discontinue use and switch to an alternate carrier oil and/or alternate ester. If anaphylaxis results, immediately seek medical attention.The anti-histamines are also only needed on a case-by-case basis. Some people are highly sensitive to certain carboxylic acids that are used to form the ester. Other people can be allergic to the oils -- though this usually results in massive inflammation that cannot be mistaken for anything else.
Ultimately it is broken down into the following set of rules.
1) Dilute compound with sterile cutting oil to get mg/ml at <=200 for long esters and <=100 for short esters.
2) Add 20% Benzyl Benzoate by volume to propionate and phenylpropionate esters.
3) Use a 23g needle to prevent high output pressure from smaller needles
4) Inject very slowly to reduce muscle fascia displacement
5) Inject frequently w/ less volume, <3ml for large muscles and <2ml for smaller muscles
6) Rotate sites every time (I use six sites)
7) Gently warm the oil to improve viscosity
Ensure that the injection is deep intra-muscular and fully through any sub cutaneous fat
You can also add:
100mg benadryl one hour prior to injection
800mg ibuprofen one hour prior to injection
However, the ibuprofen will reduce the activity of autocrine based transcription and activity of PGE and PGF so continued use can result in less muscle hypertrophy.
Warming the oil definitely improves viscosity and allows it to enter the muscle with less discomfort. When short esters, high concentrations or excess benzyl alcohol are involved, you are still going to need to dilute or add benzyl benzoate to allow for the least painful injections.
NSAID's are rarely necessary, but can help when there is fascia displacement from large volume injections.
"Injections shouldn't hurt. Don't put up with the pain. Change the compound and give feedback to your source. Market force can push the UGL's in the right direction so that everyone gets good quality product." 
