Prescription Writing

Prescription Writing

USP organization: legible (#1 concern), metric system (i.e., units-insulin)

DRUG: trade names and/or generic names

to avoid confusions due to legibility, write purpose after name (i.e., Celebrex for CV or Celebra for CNS)

Common confusions: Celebrex-Celebra, Acetazolamide-Acetohexamide, Methotrexate-Metolazone, Thorazine-Tagamet

DOSE:

.1 mg gets smudged to 1 mg therefore use 0.1 mg

1.0 mg gets smudged to 10 mg therefore use 1 mg

PATIENT: include age-weight so pharmacist will recognize a mistake in dosage

TIME: Never write "take as directed"

NO ABBREVIATIONS or LATIN:

U = units – mistaken for zero, 4, cc
ug = micrograms – mistaken for mg
QD (qd) = once daily – mistaken for QID (4x)
QOD = every other day – mistaken for QID or QD
SC/SQ = subcutaneously – mistaken for SL (sublingually)
D/C = discharge – mistaken for discontinue
HS = hours of sleep – mistaken for half strength
AU, AO, AS = both ears, right/left ear – mistaken for OU, OS, OO (eyes)

1^st order vs zero order

V = V_max C / (K_m + C), where C = concentration

1^st order:

C << K_m therefore V = V_max C / K_m
So as C increases, V increases

Zero order:

ASA, EtoH, Phenytoin
K_m << C therefore V = V_max C / C

Bioavailability

F_o (bioavailability) = AUC_oral / AUC_IV (assuming same dose for both oral and IV)

1^st pass effect: a lot of drugs do not make it past liver

CYP enzymes metabolize drugs as they pass through liver
Different CYP enzymes metabolize at different rates
CYP3A4 vs CYP2D6 = determines if pt is a slow or rapid metabolizer depending on which CYP’s they use
Therefore must adjust dosing according to the CYP that person has

Volume of Distribution (Vd)

Vd = Volume it takes to distribute drug in the plasma

C = drug amount / Vd i.e., 20 ug/ml = drug amount / 0.7 L/kg; drug amount = 14 ug/kg

Important Equations to Know

(1) Loading Dose – C = drug amount / Vd

At steady state, IN = OUT therefore:

Drug delivery = Drug elimination or

F_o × dose / dosing interval = Cl × C_ss , where Cl = clearance, C_ss = conc at steady state

Thus, C_ss = (F_o × dose / dosing interval) / Cl

(2) T½ = 0.693 × Vd / Cl

Multiple Dosing Rules

(1) extent of accumulation

increased dosing interval of drug in a day but daily dose remains same – no accumulation
increased dosing interval of drug but maintain same dose per taking – increased daily dose – accumulation

(2) time to reach plateau

increased dosing interval of drug but not daily dose – will not reach plateau any faster – depends on T½

(3) elimination dosing reaching plateau (or steady state) = maintenance dose

(4) misconceptions:

larger Vd – lower C_ss = false; C_ss depends on Cl, not Vd

C_ss is dependent on rate of absorption = false

Pharmacodynamics

competitive antagonist (K_d increased , V_max unchanged) vs non-competitive antagonist (K_d unchanged, V_max decreased )

efficacy (important quality of drug) vs potency (unimportant; amt of drug needed to reach the ultimate effect)

toxic vs therapeutic effects – therapeutic index (i.e., at 50 mg a pt experiences both therapeutic and toxic effects of a drug, however at 25 mg pt only experiences the therapeutic effects – due to the overlap of the curves for toxic and therapeutic effects)