General principles of adult dosing
Adult antibiotic regimens are more standardized than pediatric ones, since they are not usually calculated per kilogram of body weight as in Dosing children. Yet “standard” does not mean “one size fits all.” Effective therapy requires balancing three variables: the drug, the infection site, and the patient’s physiology.
The choice of dose depends heavily on the site of infection. For instance, antibiotics prescribed for pneumonia must penetrate lung tissue, while those for urinary tract infections must concentrate in urine. Some drugs achieve high levels in the bloodstream but little penetration into cerebrospinal fluid, making them unsuitable for meningitis.
Patient-specific factors add another layer. Renal function is critical, since many antibiotics are cleared by the kidneys. Dosing must be reduced in chronic kidney disease to prevent toxic accumulation. Conversely, in severe infections or obese patients, higher-than-standard doses may be required. Liver function also matters, especially for drugs like macrolides or metronidazole that undergo hepatic metabolism.
Finally, dosage depends on the form of the drug, as explored in Forms. A 500 mg oral tablet may not be equivalent in effect to 500 mg given intravenously, since bioavailability differs. Sustained-release capsules, liquid suspensions, and injections all alter how the antibiotic is absorbed and distributed.
Taken together, these principles show that even in adults, where dosing is more uniform than in children, antibiotics cannot be prescribed casually. Correct dosing is a matter of pharmacology, not guesswork.
Penicillins
Penicillins remain the archetype of antibiotic therapy, and their dosing principles illustrate the balance between simplicity and precision. In adults, regimens are generally standardized, but small variations in timing and duration make the difference between rapid cure and relapse.
For common infections such as strep throat, sinusitis, or pneumonia, Amoxicillin is prescribed in fixed doses, often two or three times daily. Frequency matters: penicillins act best when drug levels are kept consistently above the bacterial “minimum inhibitory concentration.” Missing doses or spacing them too widely reduces efficacy even if the total daily amount appears correct.
In more resistant infections, clinicians may escalate to Amoxicillin-clavulanate. Here, dosing must take into account both components: amoxicillin provides the antibacterial effect, while clavulanate protects it from bacterial enzymes. Improper ratios undermine the drug’s purpose.
Duration is equally important. A 10-day course is traditional for streptococcal pharyngitis, while shorter regimens may suffice in other conditions. Stopping early, however, risks not only relapse but also long-term complications like rheumatic fever.
Penicillin dosing demonstrates a principle repeated across classes: the right dose at the right interval is more decisive than the absolute milligram figure.
Cephalosporins
Cephalosporins are another cornerstone of adult antibiotic therapy, and dosing depends on both the generation of the drug and the site of infection. Their half-lives vary, so some require multiple daily doses, while others can be administered just once or twice per day.
Cephalexin, a first-generation cephalosporin, is typically used for uncomplicated skin and urinary infections. It must be given several times daily, reflecting its short half-life. In contrast, third-generation drugs like ceftriaxone have prolonged activity and can be dosed once daily, making them well suited for hospital use.
Another key issue is renal clearance. Most cephalosporins are excreted through the kidneys, so dose adjustment is essential in patients with chronic kidney disease to prevent accumulation and toxicity.
Duration varies by indication. Mild skin infections may require 5–7 days, while pneumonia or bone infections often demand longer therapy. As with penicillins, maintaining steady drug levels above the bacterial inhibitory threshold is vital.
Cephalosporins illustrate how drugs within one “family” can behave very differently: generation, pharmacokinetics, and infection site determine the regimen, not simply the drug name.
Macrolides
Macrolides occupy a unique place in adult dosing because of their long tissue half-life and their ability to concentrate inside cells, where certain pathogens (like Mycoplasma or Chlamydia) reside. This pharmacology allows for shorter courses compared to many other antibiotics.
Azithromycin is the most widely used macrolide, and its regimen is strikingly simple: once-daily dosing, often for just 3–5 days. The drug persists in tissues long after the last dose, ensuring continued antibacterial activity. This convenience makes adherence easier, but it also creates a temptation for misuse when patients self-prescribe.
Clarithromycin and erythromycin, in contrast, usually require twice-daily or even three-times-daily dosing. Their shorter half-lives mean that missed doses more quickly reduce drug effectiveness.
An important dosing caveat is drug interaction potential. Macrolides are metabolized in the liver and can inhibit cytochrome P450 enzymes, raising the blood levels of drugs such as statins, anticoagulants, or antiarrhythmics. This is why self-adjustment of doses is dangerous, and why therapy must be supervised.
Macrolide dosing demonstrates how antibiotic regimens are not just about killing bacteria but also about maximizing convenience without sacrificing safety.
Tetracyclines
Tetracyclines, and particularly Doxycycline, are valued for their versatility. Their dosing principles reflect both pharmacokinetics and important dietary interactions. Doxycycline is typically prescribed once or twice daily, depending on the severity of infection. Its relatively long half-life allows for simpler regimens compared with older tetracyclines, which required more frequent dosing. The drug accumulates well in tissues, making it suitable for respiratory, skin, and vector-borne infections.
A unique feature of tetracyclines is their tendency to bind with divalent cations such as calcium, magnesium, and iron. When taken together with milk, antacids, or iron supplements, absorption can be dramatically reduced. This is why the timing of doses in relation to meals is so important and why clear guidance from Forms must be followed.
Acne therapy may involve weeks of low-dose treatment, while acute infections are treated in courses of 7–10 days. Prolonged use requires monitoring for side effects such as photosensitivity or gastrointestinal irritation. Tetracycline dosing exemplifies how pharmacology and patient behavior intersect. The same milligram dose may succeed or fail depending on whether it is taken correctly.
Nitroimidazoles
Nitroimidazoles, represented by Metronidazole, are indispensable for anaerobic bacterial and protozoal infections. Their dosing rules are relatively straightforward but come with one notorious caveat: strict avoidance of alcohol during therapy and for at least 48 hours afterward, due to the risk of a severe disulfiram-like reaction.
Metronidazole is usually prescribed two or three times daily, depending on the infection site and severity. Because it is well absorbed orally and also available in intravenous form, regimens can be switched from hospital to outpatient care without major adjustment. The equivalence of forms is important, tying into the principles outlined in Forms.
Typical courses last from 5 to 10 days for infections such as bacterial vaginosis or dental abscesses, while longer regimens may be needed for intra-abdominal or bone infections. Precision in timing matters: maintaining steady blood levels ensures the continuous DNA disruption that kills susceptible organisms.
Nitroimidazoles dosing is simple in numbers but strict in rules, since frequency and alcohol prohibition are non-negotiable.
Nitrofurans
Nitrofurans, with Nitrofurantoin as their leading representative, illustrate how dosing must follow not only pharmacology but also the site of action. Nitrofurantoin is concentrated almost exclusively in urine, which makes it highly effective for uncomplicated cystitis but useless for kidney or systemic infections.
In adults, the drug is taken multiple times daily due to its short half-life. Modified-release formulations can reduce frequency, but maintaining steady urinary concentrations is still essential. Skipping doses risks subtherapeutic levels, allowing bacteria to persist and symptoms to return.
Treatment courses are typically short, from 5 to 7 days for acute cystitis. In selected cases, very low daily doses may be used as prophylaxis against recurrent UTIs, though this requires careful monitoring for long-term toxicity.
Since nitrofurantoin depends on renal excretion, dosing adjustments are critical in patients with impaired kidney function. Prescribing it in advanced renal disease not only reduces efficacy but also raises the risk of drug accumulation and adverse reactions.
Thus, nitrofurans highlight a broader rule. An antibiotic’s usefulness is bound by where it can reach effective concentrations, not just by its spectrum on paper.
Duration of therapy
The length of antibiotic courses in adults is defined by infection type and clinical response. Short regimens, such as 3–5 days with azithromycin, are adequate for many respiratory infections. Uncomplicated urinary tract infections often resolve with 5–7 days, while deeper infections like osteomyelitis or endocarditis demand weeks to months of treatment under specialist supervision.
Evidence increasingly supports tailoring duration to the minimum effective course: long enough to eradicate the pathogen, but not so long as to increase side effects or resistance. Adjustment is always made by a physician based on severity, comorbidities, and response.
Common pitfalls in adult dosing
Several recurring errors undermine antibiotic therapy in adults:
- Self-adjustment of doses. Taking “extra” tablets in severe illness or halving doses to “save pills” reduces effectiveness and fosters resistance.
- Missed or doubled doses. Skipping a dose lowers drug levels below the therapeutic threshold, while doubling raises toxicity risk.
- Improper combinations. Tetracyclines taken with dairy or iron, or fluoroquinolones with antacids, lose efficacy (see Forms).
- Alcohol or drug interactions. Metronidazole with alcohol, or macrolides with statins, can lead to dangerous reactions (see Drug interactions).
- Using leftovers. Starting an old, incomplete course often means the wrong drug or too short a regimen, compromising both safety and efficacy.
Avoiding these mistakes requires adherence to prescribed schedules and open communication with healthcare providers.