judgments, as in other matters, a court of appeals is not wedded to the district court’s reasoning. Rather, “[w]e are free, on appeal, to affirm a judgment on any independently sufficient ground.” Polyplastics, Inc. v. Transconex, Inc., 827 F.2d 859, 860-61 (1st Cir.1987); see also Chongris v. Board of Appeals, 811 F.2d 36, 37 n. 1 (1st Cir.), cert. denied, 483 U.S. 1021, 107 S.Ct. 3266, 97 L.Ed.2d 765 (1987).
Ill
It is hornbook law that to succeed on any of the legal theories pleaded here appellants must be prepared to show, at a minimum, (i) that Milissa ingested amoxicillin, which (ii) alone or in combination with phenobarbital caused T.E.N. to develop, (iii) under circumstances where provision of the medication was actionable, and (iv) that ap-pellees provided it (or, alternatively, can be held legally responsible for its provision). Appellants argue vehemently as to the fourth component — but we see no legally adequate evidence that the second criterion has been met.3 That omission, as appellees pointed out below and on appeal, is fatal to plaintiffs’ case.
Applying summary judgment jurisprudence to the elements of plaintiffs’ cause of action, it is plain that, on any theory, plaintiffs bore the burden of adducing some evidence, admissible at trial, supporting their allegation that amoxicillin caused Mil-issa’s adverse reaction. See Finn, 782 F.2d at 16; 10A C. Wright, A. Miller & M. Kane, Federal Practice and Procedure: Civil § 2727 at 133-37 (2d ed.1983). And, plaintiffs defaulted on this obligation.
We have scoured the record and find that the sole “evidence” of causation consists of an interrogatory answer supplied by the Garsides.4 This two sentence response reads in its entirety:
[Plaintiffs’ expert] Dr. Theodarides will testify that Milissa Garside suffered an adverse drug reaction which induced Toxic Epidermal Necrolysis (“TEN”). Dr. Theodarides will further testify that the causative agents of Milissa Garside’s TEN were the combination of amoxicillin and phenobarbital ingested by her.
More was required. After all, a mere promise to produce admissible evidence at trial does not suffice to thwart the summary judgment ax. Finn, 782 F.2d at 16; Soar v. National Football League Players’ Association, 550 F.2d 1287, 1289 n. 4 (1st Cir.1977).
Rule 56(c) provides that a trial court may properly “consider the pleadings, depositions, answers to interrogatories, and admissions on file, together with the affidavits, if any” in ruling on a summary judgment motion. The operative word, we suggest, is “consider.” Just as nonmovants “may not rest upon the mere allegations or denials of [their] pleading” to oppose summary judgment, Fed.R.Civ.P. 56(e), they cannot expect the court, in considering answers to interrogatories, to give weight to averments not made upon personal knowledge or those which are in a form patently inadmissible at trial.
In summary judgment proceedings, answers to interrogatories are subject to exactly the same infirmities as affidavits.
See H.B. Zachry Co. v. O’Brien, 378 F.2d 423, 425 (10th Cir.1967). Although such answers may be given effect so far as they are admissible under the rules of evidence, they should be accorded no probative force where they are not based upon personal knowledge or are otherwise deficient.
Schwartz v. Compagnie General Transatlantique, 405 F.2d 270, 273 n. 1 (2d Cir.1968);
Zachry, 378 F.2d at 425-26;
S & S Logging Co. v. Barker, 366 F.2d 617, 624 n. 7 (9th Cir.1966). Interrogatory answers,
3
Appellees contend that plaintiffs also failed to make the requisite showing on the third prong because the record is barren of evidence that amoxicillin was negligently manufactured or marketed, or that an implied warranty was violated. We do not reach the issue.
4
This response is contained in a document entitled "Plaintiffs’ Further Answers to Defendant Hoffman-LaRoche, Inc.'s Interrogatories.” The copy in the record on appeal is unsigned and undated. We assume for the sake of argument, as indicated in a transmittal letter accompanying the filing, that Milissa’s mother intended to sign the further answers.