question: Do our bodies absorb things like caffeine and alcohol at a set rate or do we absorb faster if we consume more?
comment: Essentially, we absorb them faster - up to a point. Think of digestion like pouring water through a funnel. Up to a point, things pass through into the bloodstream faster, but there's an upper limit on that speed. In a funnel, that's based on how narrow or wide the opening is - in your body, it's based on how many cells are doing the work of passing things into the bloodstream. Caffeine and alcohol don't just seep into the bloodstream, they have to be carried across the gut lining and blood vessel walls before they can make it to the blood. In pharmacology, this is called the dose-response curve, and it usually looks vaguely S-shaped: at low doses, the absorption (and the response to the drug) is quite slow, and it speeds up quickly as the dose is increased, but then it will usually level out again at high doses, and anything above that level will just stay sitting in your stomach. Concentration can also have an impact on how fast you absorb a drug (including things like caffeine and alcohol; in this sense, "drug" means anything that has an effect on your body's metabolism). This is why certain alcoholic drinks (such as poitín) are notorious for getting you drunk *the next day* - because it's at such a high concentration that your body recognises it as poison and doesn't absorb very much of it, but when you get up and drink some water, it dilutes the alcohol and a whole lot more alcohol gets into your system.
passages: There are a variety of factors that can affect the MTC, which is often the subject of clinical pharmacokinetics. Variations in MTC can occur at any point in the ADME (absorption, distribution, metabolism, and excretion) process. For example, a patient could possess a genetic defect in a drug metabolizing enzyme in the cytochrome P450 superfamily. While most individuals will possess the effective metabolizing machinery, a person with a defect will have a difficult time trying to clear the drug from their system. Thus, the drug will accumulate within the blood to higher-than-expected concentrations, reaching a MTC at a dose that would otherwise be considered normal for the average person. In other words, in a person that is intolerant to a medication, it is possible for a dose of 10 mg to "feel" like a dose of 100 mg, resulting in an overdose—a "normal" dose can be a "toxic" dose in these individuals, leading to clinically significant effects.
   • After oral ingestion and absorption, drugs while passing from the gut via the liver to blood stream are inadequately absorbed, wasted, eliminated and destroyed and therefore a reduced percentage of ingested dose reaches the blood stream resulting in lower drug levels in blood.
  
  • Therefore, more quantity of drug has to be ingested to compensate for the losses in order to achieve desired blood levels which in turn increase the cost and side effects of drugs.
   Drugs in systemic circulation have a certain concentration in the blood, which serves as a surrogate marker for how much drug will be delivered throughout the body (how much drug the rest of the body will "see"). There exists a minimum concentration of drug within the blood that will give rise to the intended therapeutic effect (minimum effective concentration, MEC), as well as a minimum concentration of drug that will give rise to an unintended adverse drug event (minimum toxic concentration, MTC). The difference between these two values is generally referred to as the therapeutic window. Different drugs have different therapeutic windows, and different people will have different MECs and MTCs for a given drug. If someone has a very low MTC for a drug, they are likely to experience adverse effects at drug concentrations lower than what it would take to produce the same adverse effects in the general populace; thus, the individual will experience significant toxicity at a dose that is otherwise considered "normal" for the average person. This individual will be considered "intolerant" to that drug.
   BULLET::::- Theophylline (4%): Relaxes smooth muscles of the bronchi, and is used to treat asthma. The therapeutic dose of theophylline, however, is many times greater than the levels attained from caffeine metabolism.
  
  1,3,7-Trimethyluric acid is a minor caffeine metabolite. Each of these metabolites is further metabolized and then excreted in the urine. Caffeine can accumulate in individuals with severe liver disease, increasing its half-life.
   Absorption is a primary focus in drug development and medicinal chemistry, since the drug must be absorbed before any medicinal effects can take place. Moreover, the drug's pharmacokinetic profile can be easily and significantly changed by adjusting factors that affect absorption.
  
  Section::::Dissolution.
  
  In the most common situation, a tablet is ingested and passes through the esophagus to the stomach. 
   The absolute bioavailability of a drug, when administered by an extravascular route, is usually less than one (i.e., "F" 100%). Various physiological factors reduce the availability of drugs prior to their entry into the systemic circulation. Whether a drug is taken with or without food will also affect absorption, other drugs taken concurrently may alter absorption and first-pass metabolism, intestinal motility alters the dissolution of the drug and may affect the degree of chemical degradation of the drug by intestinal microflora. Disease states affecting liver metabolism or gastrointestinal function will also have an effect.
   In other situations, such as intravenous therapy, intramuscular injection, enteral nutrition and others, absorption is even more straightforward and there is less variability in absorption and bioavailability is often near 100%. It is considered that intravascular administration (e.g. IV) does not involve absorption, and there is no loss of drug. The fastest route of absorption is inhalation, and not as mistakenly considered the intravenous administration.
   The route of administration is important as well. Whether a drug is ingested orally, injected into a muscle or vein, absorbed through a mucous membrane, or any of the other types of administration routes, affects how quickly the substance will be metabolized by the body and thus effects the concentration of the active ingredient(s). Dose-response curves may illustrate the relationship of these metabolic effects.
  
  Section::::Medicines.
  
  Section::::Medicines.:Over-the-counter medications.
   The rate of dissolution is a key target for controlling the duration of a drug's effect, and as such, several dosage forms that contain the same active ingredient may be available, differing only in the rate of dissolution. If a drug is supplied in a form that is not readily dissolved, the drug may be released more gradually over time with a longer duration of action. Having a longer duration of action may improve compliance since the medication will not have to be taken as often. Additionally, slow-release dosage forms may maintain concentrations within an acceptable therapeutic range over a long period of time, as opposed is quick-release dosage forms which may result in sharper peaks and troughs in serum concentrations.
   Section::::Overview.
  
  Pharmacokinetics describes how the body affects a specific xenobiotic/chemical after administration through the mechanisms of absorption and distribution, as well as the metabolic changes of the substance in the body (e.g. by metabolic enzymes such as cytochrome P450 or glucuronosyltransferase enzymes), and the effects and routes of excretion of the metabolites of the drug. Pharmacokinetic properties of chemicals are affected by the route of administration and the dose of administered drug. These may affect the absorption rate.
   Caffeine from coffee or other beverages is absorbed by the small intestine within 45 minutes of ingestion and distributed throughout all bodily tissues. Peak blood concentration is reached within 1–2 hours. It is eliminated by first-order kinetics. Caffeine can also be absorbed rectally, evidenced by suppositories of ergotamine tartrate and caffeine (for the relief of migraine) and chlorobutanol and caffeine (for the treatment of hyperemesis). However, rectal absorption is less efficient than oral: the maximum concentration (C) and total amount absorbed (AUC) are both about 30% (i.e., 1/3.5) of the oral amounts.
   For dogs a different elimination occurs, with is essentially as the unchanged drug. The reason for this difference in species, results from the dog having a higher dependence on biliary excretion of the unchanged drug for clearance. The value of the plasma clearance for dogs (0.2 ml min-1 kg-1), is 20 fold lower than the value for rats (4.1 ml min-1 kg-1), a species which is capable of metabolism. This difference leads to the different clearance route. Which becomes saturated after administration of a toxic dose of the drug.
   Due to the good permeability of blonanserin, the volume of distribution in the central nervous system is greater than that in the periphery (Vd central = 9500 L, Vd periphery = 8650 L) although it is slower to absorb into the central compartment.
  
  Blonanserin does not meet the criteria in Lipinski's rule of five. 
  
  Section::::Pharmacology.:Pharmacokinetics.:Effects of food intake.
  
  Food intake slows the absorption of blonanserin and increases the bioavailability peripherally relative to centrally. Single fasting doses are safe and the effects of feeding intake are possibly explained by an interaction between blonanserin and Cytochrome P450 3A4 in the gut.
   Some drugs, such as the prokinetic agents increase the speed with which a substance passes through the intestines. If a drug is present in the digestive tract's absorption zone for less time its blood concentration will decrease. The opposite will occur with drugs that decrease intestinal motility.
   Flucytosine is well absorbed (75 to 90%) from the gastrointestinal tract. Intake with meals slows the absorption, but does not decrease the amount absorbed. Following an oral dose of 2 grams peak serum levels are reached after approximately 6 hours. The time to peak level decreases with continued therapy. After 4 days peak levels are measured after 2 hours. The drug is eliminated renally. In normal patients flucytosine has reportedly a half-life of 2.5 to 6 hours. In patients with impaired renal function higher serum levels are seen and the drug tends to accumulate. The drug is mainly excreted unchanged in the urine (90% of an oral dose) and only traces are metabolized and excreted in the feces. Therapeutic serum levels range from 25 to 100 µg/ml. Serum levels in excess of 100 µg are associated with a higher incidence of side effects. Periodic measurements of serum levels are recommended for all patients and are a must in patients with renal damage.
   Section::::Factors influencing absorption.
  
  Along with inhalation, ingestion and injection, dermal absorption is a route of exposure for bioactive substances including medications. Absorption of substances through the skin depends on a number of factors:
  
  BULLET::::- Concentration
  
  BULLET::::- Molecular Weight of the molecule
  
  BULLET::::- Duration of contact
  
  BULLET::::- Solubility of medication
  
  BULLET::::- Physical condition of the skin
  
  BULLET::::- Part of the body exposed including the amount of hair on the skin
   Drugs with high first pass effect have a considerably higher oral dose than sublingual or parenteral dose. There is marked individual variation in the oral dose due to differences in the extent of first pass metabolism. Oral bioavailability is apparently increased in patients with severe liver diseases like cirrhosis. It is also increased if another drug competing with it in first pass metabolism is given concurrently. E.g. propranolol and chlorpromazine.
  
  Section::::See also.
  
  BULLET::::- ADME, an acronym in pharmacokinetics and pharmacology standing for absorption, distribution, metabolism, and excretion
  
  BULLET::::- Biopharmaceutics Classification System
  
  BULLET::::- Drug
  
  BULLET::::- Enteral administration
  
  BULLET::::- Partition coefficient
   Section::::Pharmacokinetics.
  
  Absorption: Ceftriaxone can be administered intravenously and intramuscularly, and the drug is completely absorbed. It is not available orally.
  
  Distribution: Ceftriaxone penetrates tissues and body fluids well, including cerebrospinal fluid to treat central nervous system infections. The average volume of distribution in adults is 5.8–13.5 liters.
  
  Metabolism: 33–67% of ceftriaxone is renally excreted as unchanged drug, but no dose adjustments are required in renal impairment with dosages up to 2 grams per day. The rest is excreted in the bile as inactive compounds from hepatic and gut flora metabolism.
   Milnacipran is well absorbed after oral dosing and has a bioavailability of 85%. Meals do not have an influence on the rapidity and extent of absorption. Peak plasma concentrations are reached 2 hours after oral dosing. The elimination half-life of 8 hours is not increased by liver impairment and old age, but by significant renal disease. Milnacipran is conjugated to the inactive glucuronide and excreted in the urine as unchanged drug and conjugate. Only traces of active metabolites are found.
  
  Section::::History.
   The prolonged phase does not contribute to drug accumulation on repeated administration but is thought to be of pharmacological significance in mediating drug effects. Renal impairment [particularly creatinine clearance  20 ml/min ( 1.2 L/h)] results in significant accumulation of enalaprilat and necessitates dosage reduction. Accumulation is probably the cause of reduced elimination in healthy elderly individuals and in patients with concomitant diabetes, hypertension and heart failure.
   The four primary systems that affect the first pass effect of a drug are the enzymes of the gastrointestinal lumen, gut wall enzymes, bacterial enzymes, and hepatic enzymes.
  
  In drug design, drug candidates may have good druglikeness but fail on first-pass metabolism because it is biochemically selective.
  
  Alternative routes of administration like suppository, intravenous, intramuscular, inhalational aerosol, transdermal and sublingual avoid the first-pass effect because they allow drugs to be absorbed directly into the systemic circulation.
   The time that it takes for serum concentrations of clinafloxacin to reach the maximum concentration (Cmax) in healthy volunteers after taking a dose by mouth is 0.7 hours. The elimination half-life in humans is 6.1 hours. Steady state levels of clinafloxacin are achieved in 3 days of twice daily dosing by mouth in healthy volunteers.
  
  About half of an administered clinafloxacin dose is found unchanged in the urine, meaning that the drug is cleared from the blood stream roughly equally by hepatic metabolism (liver-induced degradation) and renal elimination (kidney-mediated removal).
  
  Section::::Chemistry.
   Instant coffee decreases intestinal iron absorption more than drip coffee. One study estimated that, when a cup of instant coffee was ingested with a meal composed of semipurified ingredients, intestinal absorption was reduced from 5.88% to 0.97%, compared to an absorption of 1.64% with drip coffee. It was also estimated that, when the strength of the instant coffee was doubled, intestinal iron absorption fell to 0.53%. Apparently, however, there is no decrease in iron absorption when instant coffee is consumed 1 hour before a meal, but the same degree of inhibition as with simultaneous ingestion occurs when instant coffee is taken 1 hour after a meal.
   Caffeine can be quantified in blood, plasma, or serum to monitor therapy in neonates, confirm a diagnosis of poisoning, or facilitate a medicolegal death investigation. Plasma caffeine levels are usually in the range of 2–10 mg/L in coffee drinkers, 12–36 mg/L in neonates receiving treatment for apnea, and 40–400 mg/L in victims of acute overdosage. Urinary caffeine concentration is frequently measured in competitive sports programs, for which a level in excess of 15 mg/L is usually considered to represent abuse.
  
  Section::::Chemistry.:Analogs.
   BULLET::::- C is the concentration [mmol/L] or [mol/m³] (in the United States often [mg/mL])
  
  From the above definitions it follows that formula_3 is the first derivative of concentration with respect to time, i.e. the change in concentration with time.
  
  It is derived from a mass balance.
  
  Clearance of a substance is sometimes expressed as the inverse of the time constant that describes its removal rate from the body divided by its volume of distribution (or total body water).
  