Suppositories: solids that turn liquid inside
A suppository is a solid dosage form shaped to be inserted into a body cavity — usually for rectal or vaginal delivery — where it then either melts at body temperature or dissolves in the cavity's fluids to release its drug. It is the bridge between this track's solids and its liquids: solid in the wrapper, liquid where it acts. Two base philosophies mirror the ointment story. Fatty bases (cocoa butter, hard fat) are solid at room temperature but melt sharply near 37 °C. Water-soluble bases (glycerinated gelatin, PEG) do not melt but dissolve in the cavity fluid.
Keeping it safe: preservation
Any dosage form that contains water can grow microbes, and multi-dose liquids and creams are opened and re-closed many times, so they need a preservative. A good preservative is broad-spectrum, effective at low concentration, stable, non-toxic at use levels and compatible with the rest of the formulation. Common choices are the parabens, benzoic acid/benzoates and benzyl alcohol. Two practical traps: many preservatives only work in their un-ionized form, so the system pKa and pH matter; and surfactants or the oil phase can soak up the preservative, leaving too little in the water where bugs actually live.
Proving the product is good
Before a liquid or semisolid ships, the bench has to confirm it does what the label says — to the standard of the pharmacopoeia. For a suspension you check redispersibility (does one shake re-suspend the dose?) and sediment volume; for any pourable liquid you measure viscosity so it pours and doses reproducibly; for creams you watch for the emulsion separating. And for everything you must know the shelf life — the time the product stays within specification.
Shelf life is too slow to measure in real time at room temperature, so we use accelerated stability testing: store samples hot, measure how fast the drug degrades, then extrapolate to storage temperature with the Arrhenius equation. The worked estimate below shows the idea.
Arrhenius: k = A * exp(-Ea / (R*T)), so ln(k2/k1) = -(Ea/R)*(1/T2 - 1/T1)
Given (first-order hydrolysis of a drug in solution):
k(40 C = 313 K) = 0.020 per month (measured, accelerated)
Ea = 80 kJ/mol = 80000 J/mol
R = 8.314 J/(mol.K)
Want k at 25 C = 298 K (storage):
ln(k25/k40) = -(80000/8.314) * (1/298 - 1/313)
= -(9623) * (0.0033557 - 0.0031949)
= -(9623) * (0.00016083)
= -1.548
k25 = 0.020 * exp(-1.548) = 0.020 * 0.2127 = 0.00425 per month
Shelf life t90 (time to lose 10%, first order):
t90 = 0.105 / k
t90(25 C) = 0.105 / 0.00425 = ~24.7 months (~2 years)