Cosamin joint health supplements in all their versions suffer from the same problem, the proprietary blends don't actually contain anything that is going to help your joints. Even the ingredients that do have some benefits aren't contained in high enough doses to do anything. That doesn't mean that Cosamin ASU won't work as a joint health supplement at all, after all glucosamine HCL has been shown to be effective at improving joint pain. Although it should be noted that it does only really work to reduce joint pain and doesn't particularly support joint health as effectively as a lot of other supplements on the market.
The addition of chondroitin has been shown to potentially amplify the impact of the glucosamine in Cosamin ASU, but it's not much use as a standalone ingredient, but glucosamine and chondroitin sulfate are often paired up and this is fine.
The catch is of course the ASU blend which contains avocado, soy, boswellia serrata extract and green tea. Whilst there is value in the inclusion of boswellia serrata this is the only ingredient in here that's going to do anything, and that's assuming it's in the right dosage. Which in fairness, there is at least enough space in the 400mg blend for this to be the case.
So, ultimately we have a $40 supplement which contains 3 ingredients that you can find in cheaper offerings. Or for $5 a month more you can get FlexAgain, which contains these ingredients in their correct doses as well as omega 3, vitamin d & k, bromelain, resveratrol, MSM, curcumin and ginger extract all in their clinically studied effective doses to improve joint mobility, health and pain.
All in Cosamin ASU will produce some results, it just doesn't stack up against better or cheaper options.
Customers reviews of Cosamin ASU have declined in recent years, it seems that there was a formula change a couple of years ago reading the reviews. We don't have information on the old supplement formula so can't make a comment on this. However, as a result we'll only be looking at verified customers Cosamin ASU reviews from after 2016. As a result it's generally a 3* and below product. ASU as a compound is relatively unproven as a treatment for joint pain, but it seems that those taking the product suspect the amount has been reduced in the proprietary blend. Previously the product reviews were better. There were minimal complaints about Cosamin asu side effects, although it should be noted that nausea is not uncommon when taking glucosamine hcl.
The most commonly asked questions we came across whilst researching Cosamin ASU.
Cosamin DS is actually a quite a bit worse than Cosamin ASU, it's functionally the same ingredients minus the boswellia and with 6% of your daily vitamin c and 45% of you manganese requirements instead. We're not sure what Cosamin think, 6% of your Vit C dose is going to do, but it isn't very much.
Cosamin ASU should be safe for most people, however, glucosamine is often derived from shellfish so it's worth checking for allegens.
Other than that there are are minimal likely side effects aside from mild nausea.
Glucosamine HCL 1500mg - Glucosamine hcl is the purer form of the more well known glucosamine sulfate. The latter is actually available as a prescription treatment for hip and knee pain. This is well within the effective range of the ingredient and it's a good inclusion for Cosamin ASU.
Chondroitin Sulfate 350mg - Chondroitin sulfate is quite common in joint supplements although it doesn't do much to improve joint discomfort by itself, there are some studies which suggest it increases the efficacy of glucosamine.
Proprietary blend 400mg
ASU from Soy/Avocado - The problem with ASU from avocado and soy is not the most well backed of ingredients covered by only in a couple of in vitro (in petri dishes) human trials. In these studies there is some potential that it could improve type ii collagen and delay joint destruction. 
Boswellia Serrata - Boswellia needs to be at 100mg or above, ideally 200mg and in it's purest form to be effective at treating joint pain and improving joint health, realistically there's no reason that this couldn't be correctly dosed within Cosamin ASU. So this is good inclusion for the joint supplements.
Green Tea Extract - We're not really sure why this was included other than to bulk out the core ingredients.
All in all Cosamin ASU is overpriced for what it is, the fact is that it will help joint pain for a lot of people, but it's definitely not one of the best joint supplements on the market today. Cosamin ASU may have been a better supplement before the formula change, but even then it was using ingredients that are far less well backed than those available. Functionally there's 3 ingredients in this that should have an impact, but these appear in other better supplements at a comparable price point. We'd recommend that you get FlexAgain instead if you're looking for an overall joint health supplement that represents value for money.