FYI I like tort law but am not a lawyer, just a law school dropout. My view is that liability depends on the degree to which the manufacturer was in a position to be aware of the risks, and also on the relative ability of the consumer to discern such risks.

To take the latter point first, drug consumers are generally in a poor position to evaluate the risks of a given medication. If I set up a wheel manufacturing business, and sell square wheels, you are in a poor position to complain about the resulting bumpy ride [1] because the superiority of round wheels is so glaringly obvious. Unless you hold a medical or pharmaceutical degree, though, your ability to evaluate a drug's safety and efficacy is probably very limited. I'm smart enough to read and get the gist of a biochemistry paper to have an idea of what a drug does, but nowhere near knowledgeable enough to review or spot all but the most egregious errors. So while I am the sort of person who reads all the fine print on drug labels and sometimes digs into the academic research, in the end I'm taking a great deal on faith whenever I take medicine. by contrast, the drug manufacturer has not just one biochemical expert, but whole teams of them. This certainly puts them in the best position to foresee and even look for potential problems.

Now back to the first point: drug discovery is not the exact science we'd like it to be due to the gaps in our knowledge. Accordingly, drug companies must be given some leeway, compared to other manufacturers who enjoy a greater degree of foreseeability. suppose I set up a ladder company, but the ladders I manufacture employ such thin metal rungs that they cannot bear any weight above 100 pounds. They look good enough, but soon consumers begin suffering accidents when using them. I could not claim the accidents were unforeseeable since the tensile properties of metals are so well known and the tests are so easy to perform, that I should have realized the risk when designing my product, never mind before shipping it! It would be unfair to hold drug manufacturers to that degree of liability, but on the other hand a) experimental protocols exist precisely to address this problem and are widely understood within the drug industry, and b) since DES was on sale from 1941 onwards and class action lawsuits were brought against manufacturers in the late 1960s, more than enough time had elapsed for industry participants to have apprised themselves of the risk and limited the risk to consumers to a greater extent than they did.

If you read Judge Mosk's opinion, in footnote 24 [2] he summarizes a 7-point test for establishing (rather than allocating) liability. The answer you seek is there:

The suggested requirements are as follows:
1. There existed an insufficient, industry-wide standard of safety as to the manufacture of the product.
2. Plaintiff is not at fault for the absence of evidence identifying the causative agent but, rather, this absence of proof is due to defendant's conduct.
3. A generically similar defective product was manufactured by all the defendants.
4. Plaintiff's injury was caused by this defect.
5. Defendants owed a duty to the class of which plaintiff was a member.
6. There is clear and convincing evidence that plaintiff's injury was caused by a product made by one of the defendants. For example, the joined defendants accounted for a high percentage of such defective products on the market at the time of plaintiff's injury.
7. All defendants were tortfeasors.

This is a very brief abstraction of a key theory in a law review article written by a student at Fordham University, referred to in the opinion as 'the Fordham comment.' This unusually influential article [3] basically sets out a whole theory of 'enterprise liability' and damages allocation addressing why the manufacturers should share liability in the context of the DES in much greater detail, although it's rather painful to read for people who are not legal nerds as it approaches some sort of footnote event horizon. Whether the Sindell court's findings allow the legal academy to be held liable for typographically-induced brain trauma remains unresolved.

1. http://dogknobit.com/wp-content/uploads/2013/04/square-wheel...

2. http://online.ceb.com/calcases/C3/26C3d588.htm#SA000885

3. http://ir.lawnet.fordham.edu/cgi/viewcontent.cgi?article=231...