Sometimes “must” and “should” get confused.

The IndoEuropean root MED and its o-grade form MOD meant to measure or take appropriate measures, and hence to ponder, judge, or prescribe. Hence modus in Latin, meaning a measured amount of anything, a unit of measurement, a due or proper measure, and hence size, extent, or length, and a limit, boundary, or end.

In music a modus was a rhythmic pattern, beat, or metre and therefore a note or tone. Anyone who has ever tried to play Bela Bartok’s pieces for piano collectively titled Mikrokosmos—and the early pieces in the collection are simple enough even for a complete beginner—will have encountered the different musical modes in which he wrote some of them: Dorian and Phrygian, Lydian and Mixolydian (Figure 1).

Figure 1. The first three bars of “In Dorian Mode” from Mikrokosmos Book 1, by Bela Bartok; the Dorian mode uses seven notes: c, e flat, f, g, a, b flat, and c

There is hardly a word in the English language with mod- or -mod- in it that doesn’t come from modus: modal, modality, modem, modicum, and modify, commode, commodious, commodity, and accommodate. Those who are moderate and modest keep to the proper measure. The diminutive of modus is modulus, giving us module and model; and the ablative of modus is modo, just now or recently, giving us modern.

To meditate, Latin meditari, is to think in measured fashion.

By the Teutonic route we get mete, to measure out, and the adjective meet, commensurate. And related to mete is mote, from an Old English word motan, to have occasion, to be permitted or obliged, giving us the modal verb must, originally the past tense of mote.

In syntax, modality is the use of auxiliary verbs, called modal verbs, to modify the meaning of the primary verb. In English, the main (or central) modal verbs are “can”, and “could”, “shall” and “should”, “will” and “would”, “may”, “might”, and “must”; the semi-modal (or marginal modal) verbs are “ought to”, “used to”, “dare [to]”, and “need [to]”. Modal verbs have three uses.

• Epistemic modal verbs (Greek ἐπιστήμη, knowledge) express the truth or likelihood of a proposition, i.e. whether it is possible, probable, or necessarily true. For example: “Aspirin may cause Reye’s syndrome” [in general, with a particular probability]; “Aspirin might cause Reye’s syndrome” [with a particular probability, in this case]; “The aspirin must have caused Reye’s syndrome” [for certain, in this case].

• Dynamic modal verbs (Greek δύναμις power or strength) ascribe ability or volition. For example: “Aspirin can cause Reye’s syndrome” [it has the ability in general to do so, even if it didn’t do so in a particular case].

• Deontic modal verbs (Greek δέον, binding) express directives or permission. For example: “You must prescribe aspirin” [an imperative]; “You should or ought to prescribe aspirin” [preferably, unless something dictates otherwise]; “You may prescribe aspirin” [in this case or in general, if it seems appropriate]; “You might prescribe aspirin” [in general, if it were indicated]; “I don’t dare prescribe aspirin for this child” [I fear an adverse reaction]; “You need not prescribe aspirin in this case” [there is no requirement].

The difference between “must” and “should” is important. The UK’s General Medical Council (GMC) defines “must” and “should” in relation to its various pieces of guidance for doctors:

• “‘You must’ is used for an overriding duty or principle.”

• “‘You should’ [prefaces] an explanation of how you will meet the overriding duty.”

• “‘You should’ is also used where the duty or principle will not apply in all situations or circumstances, or where there are factors outside your control that affect whether or how you can follow the guidance.”

Now, the GMC’s document “Prescribing guidance: Reporting adverse drug/device and other patient safety incidents” states that “You must [emphasis added] inform the MHRA [the UK’s medicines and devices regulatory agency] about … serious suspected adverse reactions to all medicines and all [sc. suspected] reactions to products marked with a Black Triangle in the B[ritish] N[ational] F[ormulary] and elsewhere using the Yellow Card Scheme.”

However, although the word “must” suggests that reporting in this way is mandatory in the UK, it is not. It is mandatory in some countries, such as France, Denmark, and Korea. However, in Japan the proposed introduction of a mandatory system for reporting fatal adverse events led to widespread fear of prosecution and defensive medicine; physicians started to refuse to see high risk patients, and referrals to other hospitals became widespread; medical school graduates began to avoid specialties that were perceived as being legally particularly vulnerable, and various hospitals and clinics had to close. Mandatory reporting to pharmacovigilance agencies, although some might think it desirable, may not be such a good modus operandi.

Jeffrey Aronson is a clinical pharmacologist, working in the Centre for Evidence Based Medicine in Oxford’s Nuffield Department of Primary Care Health Sciences. He is also president emeritus of the British Pharmacological Society.

Competing interests: None declared.