Great question — this is exactly the kind of problem where derivative UV–Vis (1st, 2nd, 3rd derivatives) shines. Below I list several real pairs of pharmaceutical substances whose UV maxima (λmax) are reported very close to each other (difference < 5 nm or effectively overlapping in practice), give literature pointers, and finish with practical tips you can use in the lab (what derivative order to try, zero-crossing use, solvent/pH caveats).

Pairs (documented examples)

Metformin HCl — Glimepiride

Reported λmax (examples in the literature): Metformin ≈ 234 nm, Glimepiride ≈ 231–233 nm (values vary slightly with solvent/conditions). This gives an overlap within ~3 nm in many reports — widely studied with first-order and ratio/derivative methods for simultaneous estimation. (IJPER)

Aceclofenac — Diclofenac (drug / main degradation product)

Reported λmax: Aceclofenac ≈ 274–276 nm, Diclofenac ≈ 276 nm — effectively overlapping (difference often ≤ 2 nm). Several papers use 3rd-derivative and other derivative/ratio approaches to resolve them. This is a classic case where derivative spectrophotometry separates parent drug vs degradation product. (Pharmacia)

Tinidazole — Fluconazole (example from derivative-method paper)

Reported derivative/zero-crossing/peak positions in a validated first-derivative method: ~260.6 nm and ~263.9 nm for the two compounds — difference ≈ 3.3 nm (authors used first-derivative zero-crossing and peak-to-base measurements). Great real example showing how small separations are handled. (Thai Science)

Paracetamol (acetaminophen) — Aspirin (or associated co-formulants)

Reported λmax depend on solvent, but many simultaneous-estimation papers report paracetamol and aspirin maxima in the ~245–265 nm window such that in some solvent/pH combinations their peaks get quite close (solvent/pH can shift λmax by several nm). These mixtures are frequently analyzed by derivative and ratio-spectra methods. (useful example: simultaneous equations and derivative papers). (ResearchGate)

Other general examples (literature on derivative spectrophotometry shows many cases)

Reviews and method papers list many co-formulated drugs with strongly overlapping spectra (cephalosporins, NSAIDs + others) where 1st–4th derivatives or ratio-derivative methods were applied. If you need more examples in a particular therapeutic class (antibiotics, NSAIDs, antidiabetics…), I can pull a short targeted list. (ScienceDirect)

Important practical notes (why peaks can be within a few nm and how to handle it)

Solvent & pH shift peaks. λmax values depend strongly on solvent and pH. Two drugs that are 10+ nm apart in one solvent can be within