First I hope everyone eventually gets their QSC orders. I placed a very large order during their BF sale, got most from USA a few weeks later and then received the rest this past week from China, shipped out 12/31. They couldn't fulfill some of the tabs I ordered (sildenafil and 5-amino-1MQ #100 ea) so gave me a credit for them... which hopefully I will be able to use when they return (being hopeful here).
Second, very interesting discussion on aggregates. As a physician I have not really read much about this until now.
I've been using peptides for years and have frequently mixed various ones together in the same syringe to reduce number of injections, as it is pretty common practice at least in the BB community. These include HCG, GLP1s (sema first, then tirz, now reta), NA Selank Amidate, NA Semax Amidate, bremelanotide, IGF1-LR3, HCG, TB500, BPC157, SS31, MOTS-C, Cu-GHK among others.
I have never noticed any immunogenicity or had any other issues, and peptides always seem to work as intended. That said, the only ones I get a noticeable short term effect from are the IGF1-LR3 (hypoglycemia/fatigue if I don't eat enough) and of course bremelanotide

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There are a few peptides I generally take individually, including GH and GLP1s as they have sometimes become cloudy when mixed with certain others, I attributed this to different pH buffering that may pull something out of solution, but perhaps I was witnessing aggregation? Some mixes do not do this, such as GH & HCG which seem to be fine together, and sometimes I do combine those.
I have never noticed anything related to immunogenicity. Well with the exception of MOTS-C which causes some transient inflammation in nearly everyone, myself included, and I consider it a way to confirm it is real.... I attribute that to it being a mitochondrial peptide that normally is hidden from the immune system, so the body recognizes it as foreign. SS-31 and Humanin/HMG aremitochondrial peptides are also but do not seem to cause any reaction.
I guess I should start injecting everything individually... but I'm 56 and take a lot of shit to keep youthful and vigorous as my wife is 20 years younger. I am thinking aggregation risks likely increase with larger molecules such as GH - if I remember, that is the largest peptide in the body. Very small peptides like BPC probably have minimal risk. Also even injecting everything separate, I suspect some aggregates could theoretically form once they enter the lymphatics/blood... analogous to mixing in the same syringe but with a much higher volume in a slightly alkaline pH ~ 7.4.
I also mix my oils together as most do, common practice and not associated with any issue. I use a mix of anabolics rather than TRT, basically per week 133 test, 120 mast, 67 tren and 13 trestolone all enanthate per week (ZPHC Ultra Mix @ 0.19cc/day) plus another 30mg/day of something else depending on my goals at the time - currently inj YK-11 @ 30mg/day, other times sublingual anavar or IM anadrol pre workout, or my favorite tren A. This keeps my test and estrogen in range with 12.5mg aromasin twice a week, and feel great on it.