Antidepressants - Venlafaxine Withdrawal

General Comment On Drug Half-Life

If the frequency of taking a capsule/tablet (say 12 hours) is much shorter than the drug half life (say 24 hours), the capsule/tablet may be eliminated at something like three times the period of the half-life to reduce the average drug level slowly. The residual drug from the other capsules/tablets sustains the background level. If the frequency of elimination of a capsule/tablet is more than five times the half life, it will fall temporarily but then build back to its previous level.

For short half-life drugs taken twice or three times per day the drug reduction needs to be achieved in fractions of tablets with a consolidation period in between. The concentration in the body does not float down but reduces more rapidly according to the intake. Therefore the reduction here will need to be in small increments as the body levels fall so quickly. The greater the dependency the longer the consolidation will be needed between step changes in dosage and the smaller the size of the reduction acceptable. A given drug will have an inherent half-life range in the body. To achieve a longer half-life various techniques may be adopted. It is possible to have coatings on capsules/particles within capsules where the inner ones dissolve later and therefore release their drug more slowly.

Some drug particles are coated to slow down dissolution in the body. Double and triple coatings will give slower dissolution rates. However this technique can only be applied for as long as it takes the drug to pass through the body. If a longer half-life is required, a chemical intermediary can be used. Here the entire dose will be released into the blood stream but part of it will be in the form of a related drug which breaks down into the required drug over a period of several days. This can allow longer half lives to be achieved.

Antidepressant Half Life Information